IIRC these are the steps for the MMM. I needed to go back to a FW below 1.32
1. Check the tick box in the Castle Link software
2. Press ok on the pop up window
3. Connect the cable to the receiver
4. Turn on the ESC and let it arm. Red LED flashing.
5. Full throttle until the motor stops (a few seconds, no more than 5)
6. Shut everything off and plug in the Castle Link again.