There should be a thumb release on the shaft at the handle end, press with your thumb and then pull back with the other hand on the shaft. If you have a used tube in the gun, it could be that some of the caulk came out the inside of the tube and hardened so that you cannot pull back the shaft, you will now have to cut the tube off with a utility knife to remove the old dried caulk
Answers & Comments
There should be a thumb release on the shaft at the handle end, press with your thumb and then pull back with the other hand on the shaft. If you have a used tube in the gun, it could be that some of the caulk came out the inside of the tube and hardened so that you cannot pull back the shaft, you will now have to cut the tube off with a utility knife to remove the old dried caulk