Remove the hardware and hinges with an electric screwdriver. Place them in a plastic bag or container so they don't get lost. Remove the doors from the cabinets and set them on a steady work surface.
Wipe down the cabinets and doors with a soft cloth dipped in mineral spirits to remove accumulated grease and grime. Wipe dry with a dry towel.
Sand the cabinets and doors with a piece of 220-grit sandpaper. Your goal is not to remove the polyurethane but simply to rough it up a bit so the new stain penetrates the wood. Wrap the sandpaper around one of your fingers to sand crevices and detailed areas. Clean the cabinets again with a slightly damp cloth to remove sanding dust.
Stir, not shake, a can of stain and polyurethane combination sealer. Open the can and apply the stain with a brush, working in long, steady strokes. Paint the detailed areas first, then the flat surfaces. Avoid repeatedly brushing the stain, which will leave brush marks. Instead, make strokes in one direction, working in the direction of the grain.
Allow the stain to dry for several hours or overnight. Replace the hardware and reinstall the cabinet doors.