You need to find a way to hold the pulley, the blades are likely secured with a nylock nut. An impact wrench works best.

Prop a wooden block between blade and deck housing to prevent blade from turning when you are trying to loosen nut. I have a dedicated wooden block cut to correct length that I use each time I remove my blades. Be careful and watch out for slippage of block. Use the block again for tightening nut.

Leave the blade bolt in and pick up new spindle shaft kits and blades. the star is spun on the shaft. Work on removing the 7/8 nut on top to verify you are able to replace the shafts yourself.

And remember: Lefty/loosey, Righty/tighty! And yes, you must block the blades from turning, or hold the blade with one hand, and turn the large wrench or ratchet with the other hand.