To replace the PTO seal on a Massey Ferguson 135 tractor, you will need the following items: appropriate size wrench or socket and ratchet, hammer, new PTO seal, and sealant. Begin by removing the PTO lever, disconnecting the PTO shaft, and then removing the entire PTO assembly from the tractor. Carefully inspect the entire assembly for any signs of wear or damage. Once all is clear and no damage is found, you can begin to install the new PTO seal. First, use the appropriate size wrench or socket to remove the screws that hold the PTO seal in place. Once these are removed, carefully use the hammer to remove the old seal from the shaft. Apply a small amount of sealant to the new seal before installing it into the PTO shaft. Finally, use the appropriate size wrench or socket to secure the new seal in place. After everything is installed, test the PTO to ensure that it is working properly.