Unplug the box (of course) and remove the cover.

Remove the relay with a Philips #2 screwdriver and cut any burned cables away - don't try to unscrew the burned terminals, they're likely now part of the relay forever.

Remove the relay using a 11/32" hex socket on the two nuts on the sides of the relay.

Clean the affected area, removing smoke residue from the chassis and the board as much as possible.

You may need to remove the board to clean residue from around the board as well.