Did you just stick a $2 stamp on the wrong envelope? Or is that a rare Tahitian stamp you want to remove? This process works for both self-adhesive stamps and the lick-and-stick variety.
Remove the envelope's contents. Place them in a new envelope.
If the envelope is of a neutral color, fill a sink or basin with an inch (2.5 cm) of warm water and submerge the envelope.
Soak the envelope for 10 to 15 minutes. If the envelope is brightly colored, do not soak it--the color might run and ruin the stamp. Instead, hold it over a steaming kettle.
Pull the paper away from the stamp with your fingers. Remember, you want to preserve the stamp, not the paper.
Blot the stamp with paper towels.
Allow the stamp to air-dry for an hour, then use glue to affix it to the new envelope.
Or, if you plan to add it to your stamp collection, place the stamp between two pieces of cardboard and press it under a heavy book. If the stamp's back is still sticky, place the sticky side on a sheet of plastic. Allow the stamp to dry overnight.