Put dull blade in a vise and sharpen it using a file. You could try to do this by hand, but putting it in a vise is less awkward and will get the job done more quickly and accurately.
Another way to sharpen a mower blade is by using a drill and a special sharpening stone. Power it up and run it along the dull mower blade. Experts disagree on how often you should sharpen your blade. The range runs from every 2 weeks to every 6 months. To lessen the chances of a super dull blade, pick up twigs, rocks and other debris in your yard before you mow.