The easiest way is to use a hair dryer on full heat and speed. Heat for at least 2-3 minutes and gently raise one corner of the stamp with a single edge razor blade. Enough to grab the stamp and as you do gently separate it from the envelope very slowly. Place the stamp on a piece of wax paper with a paperclip until you are ready to use. Place some liquid glue on the back of the stamp and stick it on the new addressed envelope.