How to make python3 command run Python 3.6 instead of 3.5?

If you are looking for other than the accepted answer. Here is the solution that saved my life. This is to replace it with new version.

$ python3 --version
  Python 3.5.2

$ ls -lh /usr/bin/python3
  lrwxrwxrwx 1 root root 9 Mar 23  2016 /usr/bin/python3 -> python3.5

$ sudo mv /usr/bin/python3 /usr/bin/_python3
$ sudo cp /usr/bin/python3.6 /usr/bin/python3

$ python3 --version
  Python 3.6.11

You could update the default python version system-wide using update-alternatives command.

$ sudo update-alternatives  --set python3 /usr/bin/python3.6

or you can also run the following command to choose among the various python versions installed on a host.

$ sudo update-alternatives --config python

do

rm /usr/bin/python3
ln -s /usr/bin/python3.6 /usr/bin/python3

much better solution:

Damn, Python is used throughout much of Ubuntu for system scripts and software, and software relies on having Python (and the commands to start Python) in a certain spot. do back then.

rm /usr/bin/python3 
ln -s /usr/bin/python3.5 /usr/bin/python3 

create alias in ~/.bash_aliases

alias python3='/usr/bin/python3.6' 

Scripts can then start with something like:

#!/usr/bin/env python3 

Worked Perfectly...

ln -sf /usr/bin/python3.5 /usr/bin/python3