If you are using Ubuntu 14.04 or 16.04, you can use Felix Krull's deadsnakes PPA at https://launchpad.net/~deadsnakes/+archive/ubuntu/ppa:

sudo add-apt-repository ppa:deadsnakes/ppa

sudo apt-get update

sudo apt-get install python3.6

Alternatively, you can use J Fernyhough's PPA at https://launchpad.net/~jonathonf/+archive/ubuntu/python-3.6:

sudo add-apt-repository ppa:jonathonf/python-3.6

sudo apt-get update

sudo apt-get install python3.6

Ubuntu 16.10 and 17.04

If you are using Ubuntu 16.10 or 17.04, then Python 3.6 is in the universe repository, so you can just run:

sudo apt-get update

sudo apt-get install python3.6

After installation for Ubuntu 14.04, 16.04, 16.10 and 17.04

To invoke the Python 3.6 interpreter, run python3.6.

Ubuntu 17.10

Ubuntu 17.10 already comes with Python 3.6 as default. Just run python3 to invoke it.