Changing the belt on a Husqvarna/AYP self-propelled lawn mower is actually quite simple. Always disconnect the spark plug wire from the spark plug to prevent the lawn mower from starting accidentally. Either prop up the lawn mower or tip it over to its side (air filter side should be facing up). Next, remove the belt cover (baffle) there may be a couple of fasteners holding it in place. Remove the bolt in the center of the blade and lift off the blade (you may want to wear gloves for protection). You now have access to the belt and can roll it off of the pulley and remove it from the lawn mower. Replace the old belt with a new correct size belt. Install the new belt around the pulleys, put the blade back in place (correct side facing down), secure the blade bolt, and reinstall the belt cover. Lastly, reconnect the spark plug wire.
You can find the model and serial numbers located on your Husqvarna/AYP push or self-propelled lawnmower on a decal on the rear of the mower housing.
NOTE: if you need a part for the small engine in your Husqvarna/AYP push or self-propelled lawnmower, there will be a separate model/serial number for the engine. Here are the small engines that are available in your Husqvarna/AYP push or self-propelled mower. Visit the page of the engine you have to find out where the model/serial number is located for your small engine model: