There are 4 screws on the top of the amp,take those off with a phillips screwdriver.Be careful when taking the last screw out.The chassis will drop down so hold it up with your hand while taking out the last screw.You can rest the chassis on a ledge inside the amp just beneath it.Before you take the 4 screws on top off,make sure you take the piece of aluminum off that's protecting the power tubes.You will have full access to the standby switch.As far as replacing it you might want to contact Mesa support for that or wait for someone with more knowledge on here to reply.You might only be able to get a new switch from Mesa.