There are a couple of different options to do this. The first is to boil a pot of water. Remove the pot from the stove and put a pasta stainer on top of the pot. Place the letter flap side down in the strainer (20 seconds). The steam will loosen the glue. Then you can use a knife or letter opener to gently slide under the flap to separate it from the envelope. The second way to open a letter is to place it in the freezer for 3 to 4 hours. Then remove the letter from the freezer and use the blade of a knife under the flap to remove it from the envelope. When the letter warms back to room temperature you will be able to reseal it. If there is no glue left use a glue stick.