Assuming you need to replace a pulley on your lawnmower, the process is actually quite simple. You will need a few basic tools, including a socket wrench set, and it should only take you about 30 minutes to complete the task.
To begin, first make sure that your lawnmower is turned off and unplugged. Next, remove the spark plug wire so that there is no chance of the mower accidentally starting while you are working on it. With the spark plug removed, tip the mower onto its side so that you can access the underside where the pulleys are located.
Locate the damaged or broken pulley (it may be helpful to refer to your lawnmower's manual for assistance) and use a socket wrench to loosen and remove any bolts or screws holding it in place. Once the old pulley is removed, clean up any debris around the area before installing the new one.
With your new pulley in hand, line it up with the mounting holes and secure it in place using the bolts or screws you removed earlier. Be sure to tighten these securely so that there is no risk of the pulley coming loose while in use. Finally, reattach the spark plug wire and turn on your lawnmower to test it out - hopefully with smoother results thanks to your newly installed pulley!