You can install almost any other OS on your DS for free. An exception is Windows, it could be purchased at the time of ordering server only and can not be installed later.
Click "Dedicated servers" at the main dashboard, choose your server and click "To Panel".
At the DCI manager, find ISO-images tab and click Upload.
Then go to the console in the IPMI and choose Virtual Storage tab.
Choose ISO file in the Logical Drive type, click Open to go to the ISO directory. You'll find there the uploaded ISO.
Choose the ISO you need and then click Open.
Then click Plug in and OK to finish the procedure.
Reboot your server to start the Installation via ISO.