Setting your VPS operating system is possible during your freshly-bought VPS order activation.
If the VPS is already activated, you can change the OS template in a few clicks in hPanel. Just navigate to the Servers section, choose the server to which you want to apply changes:
Then, on the Server Management page, select a desired operating system and click on Save changes:
Keep in mind that changing the operating system will destroy all your current data, thus if you have some important files, make sure to download the backups 😊