Its either a 3/4 or 15/16 bolt that threads into the bottom of the spindle bolt. You need to remove it from the bottom of the deck. Do not try to remove the longer bolt that runs through the spindle and holds the spindle on top.
You will need a socket, socket wrench, breaker bar, and block of wood if you are doing it by hand. Use the block of wood to keep the blade from rotating while you loosen.
If you have a impact just hold the end of the blade and it will come right off.