I have a Yard machine riding lawn mower.Can some body tell me how to take the cutting blade out for me so I can sharpen them.

Most blades are held by a single screw with a beveled washer that keeps tension.

You need to remove the spark plug wire so the mower can't start.
A 6-pt socket with a fairly long ratchet or breaker bar. This will prevent you from rounding the corners of the bolt.

If the blade has been removed regularly, holding the blade with one hand and using the socket with the other works. If, not, you would have to wedge something like a block of wood between the blade and the deck.

A little cleaning up the threads and using anti-seize compound works wonders for easier removal the next time.

Well, first remove the spark plug(s) for safety, wear heavy gloves, and eye protection. If using hand tools, it is best to use a "breaker bar" and a socket. Use a piece of wood to hold the blade with the wood between the blade and the deck. Then carefully loosen the nut, remove the blade and sharpen. Then replace, being sure to align the knotches in the blade with the ones on the spindle assembly. And install with the slope side of the cutting edge facing up toward the top of the mowing deck.