You can renew them manually by editing within CB > User Management and there's a checkbox to renew their subscription, but this will not record a payment. To record a payment you can go through the frontend purchase process on their behalf by being logged in as a CB Moderator, navigating to their profile, and using their Subscription tab as they would. If you've an offline payment gateway you can utilize that to confirm an offline payment then in CBSubs > Baskets pay the basket to record a payment and renew the subscription.