How to switch between python 2.x and python 3.x

It is known that Python2.x can coexist with Python3.x. But how do you switch the active version in use? Simple: go to edit system environment variables (shortcut key: Windows + Break), and set the path variable as shown in the snapshot:

If you replace the path from Python27 to Python34, for instance, then you are setting the active Python version from 2.7 to 3.4.