This guide is for electronics trained technician. Execute the steps with utmost caution
Guide to open the top cover, remove of the amp module: link
Examine the amp module. Identify the location of the MOSTFET. There are 2pairs (4pcs) of MOSFET. Should any one of the burnt, all 4pcs will need to be replaced.
Older EX-M1/+ comes with electronic fuses. The fuses are likely burnt and need to be replaced too.
De-solder SPK cables, unscrews and remove the PCB from the heatsink. Cut away the MOSFET, replace with the new MOSFET. Please note the model number of the P/N MOSFET.
De-solder fuses, replace with new fuses.
Install the PCB to the heatsink. Pay attention to the insulation pads for the MOSFET.
Install the amp module back to the chassis. Connect the cables. Power up test.
It is advisable to test the amp with a pair of cheap loudspeakers. We can't too careful, can we?