How to replace the two blades on my 42" Craftsman 20 hp ? Mike

The easiest way that I have found is to remove the deck from under the tractor and turn it over where you can see the blades.
Get a short piece of 2 x 4 to keep the blade from turning and put it inside the edge of the deck where the blade will contact it and unscrew the bolt that holds the blade on. You can also use a C-Clamp if you have one available or just about anything else that will prevent the blade from turning while you are loosening or tightening it.
Put the new blade back on the same way, making sure that you have it lined up on the "star" (Most blades are stamped "grass side" so you should be able to read this when putting the blade on since the deck is upside down)
You can replace them without removing the deck using the same basic procedure but you have to raise the tractor up high enough to get to the bolts and be able to see to get the blades properly aligned when installing them.
I have personally found that the extra few minutes removing the deck is well worth the effort as it gives you the opportunity to inspect it for any other problems and to clean any built up grass clippings if necessary