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 :

update-alternatives --remove python /usr/bin/python2
update-alternatives --install /usr/bin/python python /usr/bin/python3

but I'm getting the following error for the second statement,

rejeesh@rejeesh-Vostro-1015:~$ update-alternatives --install /usr/bin/python python /usr/bin/python3
update-alternatives: --install needs <link> <name> <path> <priority>
Use ‘update-alternatives –help’ for program usage information.

The second line mentioned can be changed to

[sudo] update-alternatives --install /usr/bin/python python /usr/bin/python3 10

This gives a priority of 10 for the path of python3.

The disadvantage of alternatively editing .bashrc is that using the commands with sudo will not work.