The most effective method for opening and resealing a sealed envelope is to drop the envelope in the freezer for a few hours and then slide a knife under the seal. The cold air solidifies the seal, and the envelope can be resealed as it heats back up.
Holding the envelope over a saucepan of hot water is a second way for opening sealed envelopes. The steam from the kettle will dissolve the glue, allowing the letter to be opened without leaving any trace of tampering. If the paper appears to be overly moist, the letter should be set flat to dry. Afterward, the envelope can be resealed using a glue stick.