How you know it's a live bell?
Dis the mains and battery see if external sounder activates , if it doesn't job done as it might be dummy or faulty
If it sounds just reconnect power
There's no real way of suggesting how you should do it , some do it the H way , butcher it hang it up with a string put new panel in and move bell wire over last if you can't be bothered to power everything down
Or any other way , like did power again bell will or should shut down after 20mins