Remove the strings. The strings resonate into the pickups. This means the strings are positioned directly above the pickups and must be removed before you can do any work on your guitar. If you've just bought new strings for your guitar, you don't have to toss them out. You can reattach the strings afterward.
Open the back of your guitar. Here you will find the terminals that connect to the volume and tone controls for both pickups. If you are replacing both pickups, you will eventually remove the wires from all four of these. Don't remove anything yet; it's important to complete Step 4 before anything gets removed.
Take out the pickups. They shouldn't be fastened into the body too much. Usually, there are just a few screws on either side. Don't pull the pickup out fast. The pickups are still soldered to the wires connected to the guitar, and it is important to see how.
Note how your pickups are wired. It's probably best to keep a pen and paper nearby. If you are not experienced with changing pickups, it would be beneficial to map out how the guitar was originally connected just in case you mess up. It also pays to write down the changes you've made.
Use your soldering gun to remove the wires from the tone and volume knobs. Next, remove the old pickups. The new pickups you buy will have their own wires. Slide them through your guitar's body and connect them to the terminals using the soldering gun and solder. There is a link to different wiring schematics in the Resources section if you want to experiment.