Answer:
if its a rider you remove the deck, turn it over, clamp the old blade in place, & get a large wrench to fit the nut holding on the blade(s). if you have one, use an impact wrench. remove old blade(s) after you remove the nut. take off old blade and put on new one. replace nut and reinstall the deck.
if its a pusher just turn mower on its side and use above instructions.hope this helped and good luck