Granting subscriptions is only done from backend user management. It cannot be done from frontend. In backend edit the user in CB > User Management and you should be able to give them any plan they've access to under their Subscriptions tab.

Normal plan restrictions still apply there. So if your plan doesn't allow renewal in advance you can't renew them there.

If you need to directly edit the subscription row and change its dates that's also doable, but do not do this for any subscription that's auto recurring. To edit subscriptions navigate to CBSubs > Subscriptions then click the # under the ID column to the far right. This will let you directly edit a subscription and for example change its expiration date to whatever you like.

You'd do that from CB > User Management. Again, normal plan access checks will apply. So if they don't have access to the plan you won't be able to give the plan to them. The only exception is if a plan is toggled as backend only which will hide it from frontend and allow giving it to them from backend only.