install the latest Python.
$ brew install python
List all Python versions installed on your system.
$ ls -l /usr/local/bin/python*
Change default Python version to the latest version.
$ ln -s -f /usr/local/bin/python[your-latest-version-just-installed] /usr/local/bin/python
E.g:
$ ln -s -f /usr/local/bin/python3.9 /usr/local/bin/python
Restart terminal. exec bash -l to restart without closing terminal window
$ bash -l
Check Python version default again.
$ python --version