Recently, I was in a situation when it was needed to switch python versions for project test purposes. So I decided to write a blog post and describe a way of achieving this.

What about update-alternatives?

There is a solution (which is highly unrecommended to use) which may work if you want temporary switch python3 versions (for example python3.5 to python3.6): 

update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 1
Why this solution is bad? Because this may break soething in your system. For example, after doing this system updates stopped working for me until I switched python version back. So you should use this only for quick testing or not use at all.

Correct way: virtualenv

First of all, install virtualenv with pip if you haven't done this yet. virtualenv allows to create isolated python environments. It may also be useful if you want some application to use specific version of some package.
So you will need to create python environment for an app you are going to use specifying target python version: 

virtualenv -p python3.6 ~/MY_APP_PATH
You may also add --no-site-packages if you want to isolate your application from global site-packages as well: 
virtualenv -p python3.6 --no-site-packages ~/MY_APP_PATH
That's all setup required! You can create as many virtual environments as you need (for example you can create one for each of your applications).
To activate this virtual environment use: 
source ~/MY_APP_PATH/bin/activate
And to exit it later use: 
deactivate

That's it! It is simplier than you may expected!