If you are using anaconda to go into python environment you should have build up different environment for different python version
The following scripts may help you build up a new environment(running in anaconda prompt)
conda create -n py27 python=2.7 #for version 2.7
activate py27
conda create -n py36 python=3.6 #for version 3.6
activate py36
you may leave the environment back to your global env by typing
deactivate py27
or
deactivate py36
and then you can either switch to different environment using your anaconda UI with @Francisco Camargo 's answer
or you can stick to anaconda prompt using @Dan 's answer