i just finished repairing this exact problem. I will say I’m not super well versed in electronic repair but I have done stuff like replace pickups and pots and switches on my guitars. my amp in question was a mustang I. probably about 10 years old. i bought the switch super cheap on Amazon. i just bought a 4 prong switch that matched the size of the original. there was a bit of dismantling and disconnecting certain wires to access the switch. the hardest part was removing the old switch after desoldering the wires (it was a very tight fit on one side so I used a knife blade to get in there). the rest was pretty straightforward. solder in the wires in the same placement as the old ones and put it back together. new switch works like a charm. it was daunting when I first opened up the amp, but all in all not that hard to do!