There are two ways of installing an operating system: a premade image, and an ISO. To install via an image, please navigate to Services --> My Services and select your service to which you want to install the operating system on. Then, scroll down, and click "Re-install OS" which will take you to the page listing available operating systems. Alternatively, for potentially more operating systems, go back to the service page and instead click on "VPS Control Panel" first and then the service name, and select "Reinstall" here.
Select the operating system, and continue. Copy the password provided to you and wait approximately 10 minutes. Check the "VNC / Desktop" afterward.