Briggs & Stratton doesn't make lawnmowers. Instead, the company manufactures the small engines that power them, and the 6.5 horsepower engine is one of the most popular. It's a two-stroke engine with one spark plug that powers a single piston to operate the mower blade. The carburetor is the heart of the engine -- it's the place where the fuel mixes with the right amount of air for combustion. To ensure the air passage is always open, the engine has an air filter you have to replace regularly as part of normal maintenance.
Unscrew the air filter cover on the side of the engine with a 5/16-inch nut driver. The cover is rectangular and black, and it has slots to admit air. If the screw head has a slots for it, you can also use a No. 2 Phillips screwdriver.
Pull the top of the cover back and unhook the tabs on the bottom from the engine to remove it. The filter will come with it. Take the filter out of the cover.
Put a new filter in the cover, hook the bottom tabs onto of the cover onto the engine and close it. Drive the screw to hold it.