The opposite direction that the blade rotates. In this case I would say counter clockwise but their should be a marking somewhere on the mower that shows blade rotation direction.
Remove the bolt holding the blade to the left
Usually a short length of 2x4 can be used to wedge and hold the blade to loosen the bolt.
counterclockwise
Use a 9/16" Socket on a ratchet. There is one bolt in the center of the blade. Brace the blade with a block of wood to prevent movement. Loosen the bolt, remove the bolt and washers. Blade should come off easily now. Place your new blade in the spindle cradle and transfer the bolt and washers to it. Tighten with socket/wratchet. It's that easy! Unless, the bolt is stripped, in which case the mower deck will need to go to a machine shop to get the bolt out.
For a walk behind rotary lawn mower:Remove spark plug wire (to prevent an accidental start)Tip mower up at about a 45 degree angle (have the air filter up to prevent getting oil in it)Using a wrench turn the blade bolt counter-clock wise to remove the blade (if the blade is difficult use a block of wood to wedge between the blade and the deck to prevent it from spinningReverse the procedure to reinstall
First, REMOVE SPARK PLUG WIRE FROM SPARK PLUG, as you will be working near mower blade.Three inches from blade shaft, remove plastic bolt/plug with bare 3/8" drive ratchet extention, put in center of bolt, counter-clockwise of course. Wrench on outside of plastic bolt will work also.
counterclockwise
You probably forgot to put the shear rivets, bolt, key back in depending on the make and model of your lawn mower or you have stripped the bolt.
Set the mower on a work tabletop to access the blade(s). If it is a riding mower, put it on ramps. Locate the bolt(s) that secure the blade. Hold the blade in place with one hand while removing the bolt(s) with the other hand using a ratchet and appropriate socket or wrench. Pay careful attention to which side of the blade is down or which way is up so you can reinstall the same way,
you get the mower and flip it n it's side then you unscrew the bolt. take off the old blade and pt the new one on then you screw the bolt back on.
Install a lock washer between the head of the bolt and the flat washer that rests against the blade.
A ratchet wrench and appropriate size socket work just fine. Remove the spark plug wire before you start the job. You will have to hold the blade with one hand for leverage as you turn the ratchet with the other hand.