Park the lawnmower on a flat surface and engage the parking brake.
Turn the engine off and disconnect the spark plug.
Lift the hood of your lawnmower using the black latch on the right side.
Disconnect the lights from their wiring harness by pulling out the black plug.
Locate the two bolts that secure the hood to the body of the lawnmower. Remove them using your socket wrench and set them aside.
Remove the hood.
Check to see if your lawnmower has a heat shield. If so, detach the shield by removing the two bolts that secure it.
Locate the muffler -- a square or rectangular, silver box with an exhaust pipe leading out of it -- between the engine and the exhaust.
Remove the two bolts that hold the exhaust pipe in place using the socket wrench.
Wiggle the muffler and pull it out of the lawnmower.
Check the gasket around the exhaust port to see if it needs to be cleaned. If so, scrape it using a wire brush.
Install the new muffler and secure it with the bolts.
Replace the exhaust pipe and bolt it into place.
Replace the heat shield, if necessary.
Reattach the hood and reconnect the lights.
Reconnect the spark plug.