To fix a string that is wearing and fraying you can apply wax to it and the cables and lube to the rail. This will will allow you to increase he string life at the same time reducing friction. However, if any of the strands are broken or the strings are too far gone then it is time to buy a new string. If you do not own a press then you will need to make a trip to your pro-shop to replace the string.
Another way to try and fix your crossbow string wearing and fraying is to inspect the rail for nicks or burrs that could possibly be gnawing on your string wither every shot you take with your crossbow. You may possibly be able to repair with a fuel or emery cloth however if to a trip to the pro-shop will do the job.