Loads of solutions exist, but for changing the system default, alias is not the way to go.
$ update-alternatives --list python update-alternatives: error: no alternatives for python
ls -larth `which python`*
-rwxr-xr-x 2 root root 4.3M Nov 17 19:23 /usr/bin/python3.5
-rwxr-xr-x 1 root root 3.4M Nov 19 09:35 /usr/bin/python2.7
lrwxrwxrwx 1 root root 24 Feb 5 09:36 /usr/bin/python -> /etc/alternatives/python
So we have 2 - use these in update-alernatives
sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2
$ sudo update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.5 2 auto mode
1 /usr/bin/python2.7 1 manual mode
2 /usr/bin/python3.5 2 manual mode
Press <enter> to keep the current choice[*], or type selection number:
Done
$ python --version
Python 3.5.2
$ python2 --version
Python 2.7.12
$ python3 --version
Python 3.5.2