You as the admin can manually edit any users subscription within the admin tool. You would first need to find that user within the following admin tool:

QUICK FIND > USER SEARCH (link in top right of admin tool)
USERS / USER GROUPS > LIST USERS
USERS / USER GROUPS > SEARCH USERS

Once found you would click the view next to that user in the user search results as shown here:

Once you have clicked view you'll see the users details page and need to click the change expiration link you see in their user details as shown in the screenshot below:

You'll then see a admin tool page where you can change the subscription expiration for that user as shown here:

Make your date changes and click the save button to save your changes.