You can install an OS on your VPS for free. Exception: Windows. You can only buy Windows while ordering a server.
1. Power down the virtual machine.
2. Go to Disks.
3. Click ISO.
4. For the Image name menu, select your ISO. For the Boot priority, select To the beginning and click OK.
5. Make sure the disk image has appeared on the Virtual disks list.
6. Restart your VM.
7. Click VNC.
You'll see the boot menu of your ISO.