If you have more than one python version installed like below,
$ ls -lh /usr/bin/python* /usr/bin/python -> python2.7 /usr/bin/python2 -> python2.7 /usr/bin/python2.7 /usr/bin/python2.7-config -> x86_64-linux-gnu-python2.7-config /usr/bin/python2-config -> python2.7-config /usr/bin/python3 -> python3.6 /usr/bin/python3.6
here, as you can see we have two python versions installed, python 2.7 and python 3.6. Lets check what is our current active python version,
$ python -V Python 2.7
Now, lets say we want to change the python version from 2.7 to 3.6 , then for your current user you can do the same by only changing environment variable from .bashrc as,
$ vim ~/.bashrc alias python='/usr/bin/python3.6'
Now, either reset the current terminal by typing “reset” command or close and start another terminal and if we check the python version now,
$ python -V Python 3.6.9