Factory reset
Short answer
Push and hold both the M-button and the A.N.C. button until the LED indicator turns purple. Remove Monitor III A.N.C. from your audio devices and the Marshall Bluetooth app.
How to perform a factory reset:
Push and hold both the M-button and the ANC button for 5 seconds until the LED indicator turns purple.
Remove MONITOR III A.N.C. from your paired devices and also from the Marshall app.
Note:
Performing the factory reset will delete all user settings therefore the headphones need to be set up again.