How to make "python" command in terminal open python3 instead of python2.7 which comes with mac?
Type python3 and you will be running the new version. How you install can vary a bit, but on the current macOS you don’t need to install anything as the stub exists. On any older os I recommend installing https://brew.sh as it has a very friendly on boarding and helps you set your PATH properly to run the v3 of python. 

A shell alias is an easy way to fix this permanently. 

alias python=python3