A lawnmower blade is like any other knife; over time it will get dull. Signs that the blade needs attention are when the engine seems to be running fine, but the grass looks uneven or even pushed down rather than cut after a pass of the machine. Fortunately, like any other knife, a lawnmower blade can be sharpened or replaced. Lawnmowers are built to last for years, so blades are generally very easy to remove and service.
Remove the tension wire from the spark plug, and ground it by putting it against a metal part of the motor. This will prevent the mower from accidentally starting, which could cause serious injury. Tip the mower on its side to access the blade.
Remove the blade. Consult the manual for instructions, but often this is a simple matter of loosening the single central bolt. Use the hammer to loosen the blade if it's stuck. Clean the undercarriage carefully. This will lengthen the life of the mower, especially if it exhausts through the underdeck. Clean the rest of the outside of the lawnmower while you're at it. Many engine and other issues solve themselves if intakes are clean.
Examine the blade. If it is bent so that it won't turn evenly, don't bother trying to sharpen it; bent blades are difficult to rebalance. Likewise, if the blade is deeply gouged or rusted, it's probably not worth trying to save. New, good-quality blades are not that expensive; they retail for less than $20. If the lawnmower blade looks like too much work to repair, replace it.
If, however, the blade is just dull, carefully work the file against the edge to restore the proper angled bevel. Consult the lawnmower manual for guidance. Generally, the cutting edge should be a single bevel from the top of the blade, not from the underside.
To replace the blade, note the manufacturer and model number of the lawnmower. With this information, blades are easy to find online or by calling hardware and garden stores.
Install or reinstall the blade. Make sure the cutting edge is faced downward, toward the ground, and the lift edge is up. The manual should show this plainly and give instruction how much torque to apply to the bolt.