How to change python3 from python3.5 to python3.6

Changing the default python3 version might break many things on your system, and I'd advise against making python3 point to python3.6. Better to simply call python3.6 explicitly in those programs where it matters, and leave the python3 symlink as is.

As an alternative to aliases you can use update-alternatives:

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 1

This command will create a link in: /usr/bin/python3 to /usr/bin/python3.6.

I'm not sure if it's a good idea, maybe an update makes it broken.

An other options is creating a link to desired version of python in ~/bin then adding this path to the $PATH environment variable:

export PATH=/home/$USER/bin:$PATH

Or within your .profile:

PATH=/home/$USER/bin:$PATH