To change the length of a user's current storefront you can change their storefront subscriptions expiration date. You only need to find that user within the following admin tools:

USERS / USER GROUP > SEARCH USERS
USERS / USER GROUP > LIST USERS  

Click the "view" button next to them. Then click the "edit user data" button at the bottom of the page. Then you will see an option like the following if your client has a current storefront subscription:

If that user does not have a current storefront subscription you'll need to have the client purchase one to edit it or add one for them through the cart system. Then you can edit it.