Lexmark MS812dn / MS812dtn
Open the stapler door.
Pull down the latch of the staple cartridge holder, and then pull the holder out of the finisher.
Squeeze both sides of the empty staple case with your fingers, and then remove the staple case from the cartridge.
Insert the front side of the new staple case into the staple cartridge, and then push the rear side into the cartridge.
Push the staple cartridge into the finisher until it clicks into place.
Close the stapler door.