change the membership expiration date of a user

What is the best way to update - change a users expiration date? There is no way to change it from the membership --> all members page. Expiration date is the only thing that cannot be changed, but everything else can, seems weird. Any suggestions because I periodically will offer a free "premium" membership to a user and I would have to be able to set a time limit on how long they get it until they pay, or even just adjust a users expiration date maybe to compensate for site down time.