From Multiplay Wiki
To renew your server go to your multiplay account and go to Online services. In your list of services there is an option to renew that service which is located right of the service. You can Renew Up to 7 days before server expiry.
Alternatively there are notifications in Clanforge that are located under the option tabs in the left sidebar which appear when you have 5-7 days before expiry of that service.