Most Fender Stratocasters have a single coil in all three positions -- the bridge, middle and neck. You can replace the bridge single-coil pickup with a humbucker by replacing the pick guard as well as the pickup itself. This configuration will give you a thicker tone for playing rhythm guitar and is easy to perform as long as you have some soldering skills and are comfortable with the wiring that is involved.
Purchase a pick guard that will accommodate a humbucker pickup. Aftermarket guitar-parts companies sell pick guards with extra-large routes for humbuckers. You can buy one of the same colour that you have or experiment and try a different colour combination.
Unscrew the old pick guard from the Strat body. Unscrew all 11 screws using a small Phillips-head screwdriver and set them aside, as you may want to keep the old pick guard if you ever want to switch back. Carefully pull up on the pick guard and flip it over to reveal the electronics and pickups, which will be attached.
Unscrew the three pickups from the pick guard as well as the potentiometers by unscrewing the screws for all of the components from the top of the pick guard. Also loosen the washers on each of the three potentiometers.
Set the old pick guard aside and prepare the new one and the new humbucker. Screw the neck and middle pickups into the new pick guard as well as the five-way selector switch and three potentiometers. Place the potentiometers in their correct place as their wires will only be long enough to accommodate their original placement.
Unsolder the original bridge pickup. Unsolder where the wires meet the old pickup so that you can reuse these wires for the humbucker. Set the old bridge pickup aside for later use if you decide to go back to an all-single-coil set-up.
Solder the new humbucker using the old wires. Solder the red wire from the old electronics to the red wire on the humbucker and black to the black wire as well. Screw the humbucker into its position on the new pick guard, which is routed to fit the larger pickup.
Plug the guitar into an amp or a tuner and set the five-way selector switch to bridge-pickup-only, which will be the position farthest from the pickups. Tap on the humbucker with a screwdriver and listen for a sound. This will let you know that the humbucker has been wired properly and is ready to be used. If using a tuner, a signal should be registered on the tuner to be sure the humbucker is working.
Screw the pick guard back into place using the included 11 screws. Test the guitar by playing it through an amplifier in the bridge-pickup-only position.