Bike pedals are among the most important parts of a functioning bicycle. If they become damaged or worn out, they need to be replaced. Unlike other parts of the bike, however, you can replace bike pedals very easily, using nothing more than a few simple tools.
Grasp the pedal mounting with the wrench and turn the wrench towards the back of the bike (clockwise for the bike's left-hand side, counterclockwise for the right). This should loosen the old pedal.
Unscrew the pedal from the mount by hand and remove it once you have sufficiently loosened it with the wrench.
Repeat the process with the pedal on the other side of the bike. You will still turn the wrench towards the back of the bike, but since you're now on the opposite side, the wrench will turn in the opposite direction (clockwise for the left side, away from the gear mechanism, counterclockwise for the right side closest to the gears).
Check the clip-in mechanism of the new pedals to make sure you are placing them on the proper side of the bike. Many pedals are specifically right side or left side, meaning you need to place them in the proper location. Some don't have such specifications and can be attached to either the right or left side.
Grease the threads on the new bike pedals to make sure they fit onto the crank smoothly. Otherwise, they may rust up if they get wet and freeze to the crank, preventing you from pedaling.
Thread the new pedal onto the crank arm and twist it forward towards the front of the bike to lock it in place (clockwise for the right-hand pedal, counterclockwise for the left). You can do this with your hand to start with, then use the wrench to tighten it fully into place.
Repeat the process with the second pedal, using the wrench to tighten it into place once you have threaded it in by hand.
You probably want to immobilize the pedal cranks in some way while you replace the pedals. You can to this by using your free hand to steady it or simply by lying the bike on its side. Otherwise, the crank will turn as you try to work. Bicycle shops sell pedal wrenches specifically designed to replace bike pedals. While normal wrenches can do the job, you tend to get more leverage in this operation when you use a pedal wrench.