The simplest and easiest way to repair the shell is to make sure your water contains calcium in the right amount.
Most snails living in water need hard water (water containing dissolved minerals and calcium salts); one way to do this is to add GH salt, another is to add crushed sea shells.
If the mineral content of your water is good the snails will repair their shell damage by themselves - and this does not take long, you will see great improvement within a few weeks. This is the case if the damage is at the opening of the snail's shell, so if the damage is in the older part of the shell in an area where the snail cannot reach it, you need to do the repair yourself.
More information can be found here under the section "water". Be sure the water have a low content of metals, as they are toxic to snails and most other life in your tank.
If there is damage to the older part of the shell or at the top of the shell, you need to do the repair yourself by cleaning the area and then applying some form of clear nail varnish or fast hardening epoxy glue. The important thing when you do this is to avoid the chemicals coming in contact with the snail's body. This can be done by putting a part of an eggshell on the snail shell before you apply the epoxy or nail varnish on top.
You can find more information about this here. This link was provided by James Jenkins; credit to him for this.