The M6 repeater can be reset using the Reset button on the back, as from the picture below

This is how the Reset button works exactly

• If you keep the button pressed for less than 2 seconds, nothing will happen
• If you keep it press more than 2 seconds and less than 6, the M6 repeater will reset (delete its DECT registration on the current base or repeater, and start a new registration process)
• If you keep it pressed more than 6 seconds and less than 18 seconds, nothing will happen
• If you keep it pressed for more than 18 seconds and less than 60 seconds, that will enable/disable repeater monitor beep tone in the handset during a call