As good as it sounds unfortunately you cannot reuse a stamp. A stamp that has been used to deliver mail through the USPS is unable to be used again to send another letter. When a letter or package is sent through the mail it is tagged with a ultraviolet ink or covered with a franking mark that sorting machines can detect when a postage stamp has been used. Reusing postage stamps is illegal, not worth the trouble and very likely will not work. For 55c it is far easier to just buy a new stamp. If you’ve got old stamps considered collecting them as a hobby or selling them to an enthusiast.