I have a 17.5 hp mtd yard machine. Can anyone tell me how to take the mowing deck off so I can change the blades?

You do not need to remove the deck to change the blade but you do need a shirt that your wife won't care about. Raise the deck all the way, block the blade with a 2x4 or blade holder. You should be able to reach the bolts for each blade. Still too tight, jack the front end up with jack stands