The best way to open a sealed envelope and reseal it is to place the envelope in the freezer for a few hours and then run a knife under the seal. The cold air hardens the seal and once it warms back up, the envelope can be resealed.
Another method of opening a sealed envelope involves holding the envelope over a kettle of boiling water. The steam from the kettle will dissolve the glue and allow an individual to open the letter without any evidence it has been tampered with. If the paper seems too wet, the letter should be laid on a flat surface to dry. The envelope can then be resealed using a glue stick.