popos(ubuntu) 19.10执行更新时,提示找不到源:

sudo apt-get update
忽略:1 http://us.archive.ubuntu.com/ubuntu eoan InRelease                      
忽略:2 http://us.archive.ubuntu.com/ubuntu eoan-security InRelease             
忽略:3 http://us.archive.ubuntu.com/ubuntu eoan-updates InRelease  
忽略:4 http://us.archive.ubuntu.com/ubuntu eoan-backports InRelease
错误:5 http://us.archive.ubuntu.com/ubuntu eoan Release            
  404  Not Found [IP: 91.189.91.39 80]
错误:6 http://us.archive.ubuntu.com/ubuntu eoan-security Release   
  404  Not Found [IP: 91.189.91.39 80]
错误:7 http://us.archive.ubuntu.com/ubuntu eoan-updates Release    
  404  Not Found [IP: 91.189.91.39 80]
错误:8 http://us.archive.ubuntu.com/ubuntu eoan-backports Release              
  404  Not Found [IP: 91.189.91.39 80]
...
  无法连接上 apt.pop-os.org:80 (120.240.95.35)。 - connect (111: 拒绝连接)
正在读取软件包列表... 完成      
E: 仓库 “http://us.archive.ubuntu.com/ubuntu eoan Release” 没有 Release 文件。
N: 无法安全地用该源进行更新，所以默认禁用该源。
N: 参见 apt-secure(8) 手册以了解仓库创建和用户配置方面的细节。
E: 仓库 “http://us.archive.ubuntu.com/ubuntu eoan-security Release” 没有 Release 文件。
N: 无法安全地用该源进行更新，所以默认禁用该源。
N: 参见 apt-secure(8) 手册以了解仓库创建和用户配置方面的细节。
E: 仓库 “http://us.archive.ubuntu.com/ubuntu eoan-updates Release” 没有 Release 文件。
N: 无法安全地用该源进行更新，所以默认禁用该源。
N: 参见 apt-secure(8) 手册以了解仓库创建和用户配置方面的细节。
E: 仓库 “http://us.archive.ubuntu.com/ubuntu eoan-backports Release” 没有 Release 文件。
N: 无法安全地用该源进行更新，所以默认禁用该源。
N: 参见 apt-secure(8) 手册以了解仓库创建和用户配置方面的细节。

查看下系统版本信息:
pop-os:/etc/apt$ uname -a
Linux pop-os 5.3.0-7625-generic #27~1576774560~19.10~f432cd8-Ubuntu SMP Thu Dec 19 20:35:37 UTC  x86_64 x86_64 x86_64 GNU/Linux

当前popos已更新到20.10版本,早期源需要使用old-release.ubuntu.com,也就是需要把源配置文件/etc/apt/sources.list中的us.ubuntu.com替换成old-release.ubuntu.com.当然也可以替换为相应国内源地址,执行命令替换,替换前最好备份下//etc/apt/sources.list文件

pop-os:/etc/apt$ sudo sed -i 's/us.archive/old-releases/g' /etc/apt/sources.list

替换后再执行更新源索引命令就可以了: 

pop-os:/etc/apt$ sudo apt update -m
...
获取:6 http://old-releases.ubuntu.com/ubuntu eoan InRelease [255 kB]           
获取:7 http://old-releases.ubuntu.com/ubuntu eoan-security InRelease [97.5 kB] 
获取:8 http://old-releases.ubuntu.com/ubuntu eoan-updates InRelease [97.5 kB]  
获取:9 http://old-releases.ubuntu.com/ubuntu eoan-backports InRelease [88.8 kB]
获取:10 http://old-releases.ubuntu.com/ubuntu eoan/universe Sources [9,668 kB] 
命中:11 http://apt.pop-os.org/proprietary eoan InRelease                       
获取:12 http://old-releases.ubuntu.com/ubuntu eoan/main Sources [833 kB]       
获取:13 http://old-releases.ubuntu.com/ubuntu eoan/multiverse Sources [186 kB] 
获取:14 http://old-releases.ubuntu.com/ubuntu eoan/restricted Sources [5,532 B]
获取:15 http://old-releases.ubuntu.com/ubuntu eoan/main i386 Packages [950 kB] 
获取:16 http://old-releases.ubuntu.com/ubuntu eoan/main amd64 Packages [969 kB]
获取:17 http://old-releases.ubuntu.com/ubuntu eoan/main Translation-zh_CN [64.9 kB]
获取:18 http://old-releases.ubuntu.com/ubuntu eoan/main Translation-en [505 kB]
获取:19 http://old-releases.ubuntu.com/ubuntu eoan/main amd64 DEP-11 Metadata [510 kB]
获取:20 http://old-releases.ubuntu.com/ubuntu eoan/main DEP-11 48x48 Icons [95.4 kB]
获取:21 http://old-releases.ubuntu.com/ubuntu eoan/main DEP-11 64x64 Icons [173 kB]
获取:22 http://old-releases.ubuntu.com/ubuntu eoan/main DEP-11 64x64@2 Icons [2,962 B]
获取:23 http://old-releases.ubuntu.com/ubuntu eoan/main DEP-11 128x128 Icons [401 kB]
获取:24 http://old-releases.ubuntu.com/ubuntu eoan/main amd64 c-n-f Metadata [29.7 kB]
获取:25 http://old-releases.ubuntu.com/ubuntu eoan/restricted amd64 Packages [16.3 kB]
获取:26 http://old-releases.ubuntu.com/ubuntu eoan/restricted i386 Packages [9,904 B]
获取:27 http://old-releases.ubuntu.com/ubuntu eoan/restricted Translation-zh_CN [1,360 B]
获取:28 http://old-releases.ubuntu.com/ubuntu eoan/restricted Translation-en [5,196 B]
获取:29 http://old-releases.ubuntu.com/ubuntu eoan/restricted amd64 c-n-f Metadata [364 B]
获取:30 http://old-releases.ubuntu.com/ubuntu eoan/universe amd64 Packages [8,798 kB]
...
获取:92 http://old-releases.ubuntu.com/ubuntu eoan-updates/main DEP-11 128x128 Icons [112 kB]
获取:93 http://old-releases.ubuntu.com/ubuntu eoan-updates/main amd64 c-n-f Metadata [9,812 B]
获取:94 http://old-releases.ubuntu.com/ubuntu eoan-updates/restricted i386 Packages [6,352 B]
获取:95 http://old-releases.ubuntu.com/ubuntu eoan-updates/restricted amd64 Packages [41.9 kB]
获取:96 http://old-releases.ubuntu.com/ubuntu eoan-updates/restricted Translation-en [8,108 B]
获取:97 http://old-releases.ubuntu.com/ubuntu eoan-updates/restricted amd64 c-n-f Metadata [320 B]
获取:98 http://old-releases.ubuntu.com/ubuntu eoan-updates/universe amd64 Packages [225 kB]
获取:99 http://old-releases.ubuntu.com/ubuntu eoan-updates/universe i386 Packages [217 kB]
获取:100 http://old-releases.ubuntu.com/ubuntu eoan-updates/universe Translation-en [89.4 kB]
获取:101 http://old-releases.ubuntu.com/ubuntu eoan-updates/universe amd64 DEP-11 Metadata [63.8 kB]
获取:102 http://old-releases.ubuntu.com/ubuntu eoan-updates/universe DEP-11 48x48 Icons [23.6 kB]
获取:103 http://old-releases.ubuntu.com/ubuntu eoan-updates/universe DEP-11 64x64 Icons [45.5 kB]
获取:104 http://old-releases.ubuntu.com/ubuntu eoan-updates/universe DEP-11 64x64@2 Icons [29 B]
获取:105 http://old-releases.ubuntu.com/ubuntu eoan-updates/universe DEP-11 128x128 Icons [93.3 kB]
获取:106 http://old-releases.ubuntu.com/ubuntu eoan-updates/universe amd64 c-n-f Metadata [7,908 B]
获取:107 http://old-releases.ubuntu.com/ubuntu eoan-updates/multiverse amd64 Packages [1,172 B]
获取:108 http://old-releases.ubuntu.com/ubuntu eoan-updates/multiverse i386 Packages [1,340 B]
获取:109 http://old-releases.ubuntu.com/ubuntu eoan-updates/multiverse Translation-en [632 B]
获取:110 http://old-releases.ubuntu.com/ubuntu eoan-updates/multiverse amd64 c-n-f Metadata [112 B]
获取:111 http://old-releases.ubuntu.com/ubuntu eoan-backports/main Sources [1,080 B]
获取:112 http://old-releases.ubuntu.com/ubuntu eoan-backports/universe Sources [2,580 B]
获取:113 http://old-releases.ubuntu.com/ubuntu eoan-backports/main amd64 Packages [756 B]
获取:114 http://old-releases.ubuntu.com/ubuntu eoan-backports/main i386 Packages [756 B]
获取:115 http://old-releases.ubuntu.com/ubuntu eoan-backports/main Translation-en [324 B]
获取:116 http://old-releases.ubuntu.com/ubuntu eoan-backports/main amd64 c-n-f Metadata [220 B]
获取:117 http://old-releases.ubuntu.com/ubuntu eoan-backports/restricted amd64 c-n-f Metadata [116 B]
获取:118 http://old-releases.ubuntu.com/ubuntu eoan-backports/universe i386 Packages [3,896 B]
获取:119 http://old-releases.ubuntu.com/ubuntu eoan-backports/universe amd64 Packages [3,900 B]
获取:120 http://old-releases.ubuntu.com/ubuntu eoan-backports/universe Translation-en [2,108 B]
获取:121 http://old-releases.ubuntu.com/ubuntu eoan-backports/universe amd64 DEP-11 Metadata [9,100 B]
获取:122 http://old-releases.ubuntu.com/ubuntu eoan-backports/universe DEP-11 48x48 Icons [2,809 B]
获取:123 http://old-releases.ubuntu.com/ubuntu eoan-backports/universe DEP-11 64x64 Icons [3,943 B]
获取:124 http://old-releases.ubuntu.com/ubuntu eoan-backports/universe DEP-11 64x64@2 Icons [29 B]
获取:125 http://old-releases.ubuntu.com/ubuntu eoan-backports/universe DEP-11 128x128 Icons [6,807 B]
获取:126 http://old-releases.ubuntu.com/ubuntu eoan-backports/universe amd64 c-n-f Metadata [248 B]
获取:127 http://old-releases.ubuntu.com/ubuntu eoan-backports/multiverse amd64 c-n-f Metadata [116 B]
已下载 72.6 MB，耗时 1分 22秒 (889 kB/s)                                       
正在读取软件包列表... 完成
正在分析软件包的依赖关系树       
正在读取状态信息... 完成       
有 38 个软件包可以升级。请执行 ‘apt list --upgradable’ 来查看它们。
..