Murray riding mowers, like other lawn mowers, use rotating blades to cut the grass. The blades rotate rapidly, smacking into the grass blade. The sharp edge shears through the blade. After some time, the blade will gradually become dull. When the blade dulls, the cut isn't as clean. A ragged-cut edge on the grass blade allows fungus and diseases to get into the grass easier. With the right tools, it is fairly easy to remove the blades from a Murray riding mower.
Disconnect the spark plug wire. This will prevent the engine from starting up when you turn the blade.
Remove the mower housing. Check with your individual model's operation manual to see how the mower housing is removed.
Take your piece of two-by-four and wedge it in between the blade and the mower housing.
Use an adjustable wrench and remove the nut the holds the blade to the mower housing. The blade will slide off of the blade shaft. Save all the washers that come off with the blade. You will probably have to reinstall them with the new blade.
Repeat step 4 until all the blades are removed from the mower housing.