I was trying to set default python version to python3 in Ubuntu 16.04. By default it is python2 (2.7). I followed below steps :
Set priority for default python in Linux terminal by adding this:
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 10
sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 1
Here, we set python3 to have priority 10 and python2 to priority 1. This will make python3 the default python. If you want Python2 as default then make a priority of python2 higher then python3