Shut off the electrical current to the electrical outlets using the electrical panel. Remove the covers from the outlets using a flat head screwdriver.
Remove the screws that hold the electrical outlet boxes in the wall. Pull the electrical boxes out of the cutouts in the wall and work the electrical outlets through the openings in the middle of the electrical boxes to remove the electrical boxes completely.
Measure the depth of the electrical outlet cutouts after you have finished installing the back splash. Trim the electrical box extenders with scissors to match the new depth. Insert the electrical outlets through the openings in the middle of the box extenders. Drive the screws through the holes in the box extenders, electrical outlets and the holes in the wall.
Place the electrical outlet covers over the outlets and drive the screws through the covers and into the holes in the outlets. Restore the electrical flow to the outlets with the electrical panel.