This will download an installer, and when you run that file, you’ll be taken through the next steps. Seeing as you want to know how to update Python, chances are high you already have an older version ...
Python 2.x is installed in macOS by default. Even macOS 10.13 High Sierra has Python 2.7.4 as its only version. As the Python website says, “Python 2.x is legacy, Python 3.x is the present and future ...