首先在phpstudy中安装composer。

然后找到php的安装目录，我的是在 D:\phpstudy_pro\Extensions\php\php7.3.4nts

找到 composer 的安装目录，我的是在 D:\phpstudy_pro\Extensions\composer1.8.5

修改一下这个目录中的 composer.bat 文件为：

@echo OFF
:: in case DelayedExpansion is on and a path contains ! 
setlocal DISABLEDELAYEDEXPANSION
D:\phpstudy_pro\Extensions\php\php7.3.4nts\php "%~dp0composer.phar" %*

然后运行升级命令： composer self-update

成功了！

D:\phpstudy_pro\Extensions\composer1.8.5>composer self-update
Updating to version 2.0.9 (stable channel).
   Downloading (100%)
Use composer self-update --rollback to return to version 1.8.5

再试试：

D:\phpstudy_pro\Extensions\composer1.8.5>composer self-update
You are already using the latest available Composer version 2.0.9 (stable channel).