This is a common new-power-user mistake. DO NOT CHANGE the system-provided version of Python – doing so will break your system. Many important services depend upon Python, and a few break when versions change. You can run with different versions all you like (tools like venv and conda are good for that). Tips for success: DON’T remove or replace the system-provided Python, and DON’T change the /usr/bin/python3 symlink to point to a different version of Python. Within those two minor constraints, the world is your oyster.
Ubuntu 18.04 current Python3 version.
python3 --version Python 3.6.9