I have an above ground pool with rusting walls and 2 small holes . Howcan I fix?

Most above ground pools use aluminim so I think this is corrosion rather than rust. In any case, once you have holes it is not recommende that you try to patch them. You patch would be exterior and the water pressure will still be interior. If you are really in a pinch and can't handle the expense of replacement right now you could try one thing but no guaranyees on this lasting. Remove the top rail and pull up the liner so you can slip some thin plastic sheeting between the liner and the pool wall. I am talking about plastic sheeting the thickness of a matchbook cover. You may have to serach around to find a good source but hardware store and even a stationary store may sell booklet covers made from plastic. Galvanized sheet metal would also work but be careful to not tear the liner.

Can you safely replace the top rails on the pool while the water is still in the pool? We don't want to take all the water out. We have a 30 foot round pool and that's a lot of water.