Quoted from Cash_Riprock:
If the top is mushroomed you'll have to take it out thru the top of pf...most likely.
My magnet came mushroomed also. Problem is the first ball I played got stuck behind it so I decided to remove it. Removing it from the top is the only way it will come out without damaging that thin area of the playfield and requires removing just about everthing to get to it. It is wise to take pics along the way as it is almost a total tear down.
Removing the back panel is necessary as well, so that should be one of the first things you do.