The frequency with which you will need to replace your Husqvarna lawn mower blade will largely depend on the amount of mowing that you do. One of the bet ways to tell if a lawn mower blade needs changing is through physical inspection. Each time you take the blade off to sharpen it, examine the blade for nicks too deep to be filed away, bends in the blade and extensive wear and tear. If any one of those are present, it is time to change the lawn mower's blade.
Place the Husqvarna lawn mower outside. Turn the engine on and let the mower run in place until all of the gasoline is gone.
Look on the top of the lawn mower near the engine and locate the spark plug. Remove it by pulling it out with your fingers. If you are unsure of its location on your particular model, consult the owner's manual.
Turn the Husqvarna lawn mower on its right side.
Wedge a 2-by-4 between the blade of the mower and the bottom of the machine to stop it from moving while you dislodge the blade.
Remove the nut in the center of the lawn mower blade with a long handle wrench (in some models, the blades are affixed with three nuts. In this case, use the wrench to remove the outside nuts first and then the center nut).
Slide the old lawn mower blade off of the bolt(s).
Replace the lawn mower blade with a new one by sliding it back onto the bolt(s).
Secure the nut(s) onto the bolt(s) with a torque wrench. The nut(s) will need around 60 pounds of torque to be secure.
Right the lawn mower, replace the spark plug and re-fill the tank with gasoline.