Trying to change belt for push mower , cant figure out how to get it off shaft , plastic guard off shaft , this belt is for the self propelled front wheels

To change the belt, you have to DISCONNECT THE SPARK PLUG first, take off the blade AND adapter off the engine shaft. (Note- it is probably going to be stuck on the shaft so spray it up with penetrating oil and tap it back and forth with a hammer to loosen it. I had to use a puller to get it off.) Next, take off the plastic cover that covers the belt and put it around the pulley feeding it through the opening to the other pulley on the transmission. Coat the engine shaft with Neverseez or something similar so next time you wont have to fight with it to get the adapter off. Replace all parts in reverse order and function test mower.

If you have a large enough work bench it will make this a lot easier to do but it can be accomplished on the ground.
Lay your mower back on its handle bar and brace it with something heavy enough to hold it in place. This will keep your oil from running out into the air filter and/or into the cylinder as opposed to turning it over on its side.
Spray a bit of good penetrating oil around the shaft ABOVE the blade adapter and let it soak for a bit.
Remove the bolt holding the blade to the blade adapter then using a small hammer, gently tap downward on the blade adapter as you pull on it at the same time. Avoid striking it with enough force to break it. They usually slip off with just a little bit of effort.
Now you should be able to remove the plastic belt guard from the shaft and access the belt.
If you can't work it off the shaft pulley, remove it from the transmission pulley first. You will also need to remove the cover from the top side of the mower in order to replace the belt.
You didn't give a model number so I don't know if there are any screws securing the bottom guard but most of them just sit there on top of the blade adapter and slide into a cut-out area of the deck