Turn the instrument upside down, if possible, so that the thick collar at the end of the lead pipe is accessible. The mouthpiece will be inserted into the lead pipe for playing.
Hold the instrument securely and locate the thicker mounting collar on the end of the lead pipe. Put on a rubber glove, if desired, and grip the mouth piece.
Tap the thick collar on the end of the lead pipe with an old drum stick or similar sized wooden dowel. Work your way all the way around the collar and do not hit the lead pipe.
Turn and pull on the mouthpiece as you continue to tap on the collar. Gradually increase how hard the collar is struck with the stick and continue to pull and turn.
Increase the frequency that the collar is struck with the stick or dowel to increase the vibrations and work the stuck mouthpiece free.