You want to score the neck lightly with a knife, and I mean lightly... preferably with a swiss army knife until you see a white line appear in the glass.
Wrap the amp in a paper towel and insert it in to a pen cap. Then snap the neck off, but make sure you have your hand covered because if you didnt score the neck enough it will bust a wound in your hand
**Or... by an amp opener! Good luck B Lee