You can install almost any other OS on your VPS for free. An exception is Windows, it could be purchased at the time of ordering server only and can not be installed later.
How to install OS
Power down the virtual machine first.
Select Disks.
Click ISO.
Select your ISO in drop-down Image Name menu. Set boot priority "To the beginning" and click OK.
It will appear in the Virtual disks list.
Then start VM.
And click VNC.
Afterwards you'll see the boot menu of your ISO.