Leave door open -- hold the green “GO” button down which is located on the top of the printer until all lights cycle through and stay lit.

Release the button--close the door--lights should go out

Note:  The DR350 Drum can not be reset until the "Replace Drum Warning" has been received.  You can not remove a Drum unit half way through its life, then install a new Drum Unit and expect to reset the counter.  It can not be done.

Mfc-7420 w/LCD

Open the front door.
Press the Options key on the control panel until the display shows "REPLACE DRUM - 1.YES 2.NO".
Press 1 to reset the drum count.
Close the front door.