How to select Python version in PyCharm?

File -> Settings

Preferences->Project Interpreter->Python Interpreters

If it's not listed add it.

PyCharm 2019.1+

There is a new feature called Interpreter in status bar (scroll down a little bit). This makes switching between python interpreters and seeing which version you’re using easier.

Enable status bar

In case you cannot see the status bar, you can easily activate it by running the Find Action command (Ctrl+Shift+A or ⌘+ ⇧+A on mac). Then type status bar and choose View: Status Bar to see it.

I think you are saying that you have python2 and python3 installed and have added a reference to each version under Pycharm > Settings > Project Interpreter

What I think you are asking is how do you have some projects run with Python 2 and some projects running with Python 3.

If so, you can look under Run > Edit Configurations