Does a system even exist for upgrading stuff? You could always make up your own, or just think about it as if it were real life. You can go about it from case to case, since you are the DM.
Just ask yourself: how much would a certain modification cost? (Gauge it based on what the original weapon piece cost, or what that type of technology in your setting costs, and labor or whatever to install it). What features would the modification add? If it is a gun, and someone wants to modify it, you can think of a few options for it that are available at the store or whatever, so if you think of more later, what you thought of previously was just for that store. For example, say that someone has a revolver and wants to upgrade. You would think of some options, like, new grip, which would add a point to the hit chance of a shot, or something more expensive, that was saved before the apocalypse, a heads-up display eyepiece, which would have a laser-type of sensor on the gun, and the eyepiece would display you where your gun is aiming. This would make it so you could just whip out your gun and not worry about stance or anything, just getting to a stable position with your hand. Such a device would add more to the hit chance of a shot, or because its appeal is to just speedily know where you will hit, even with one hand on the weapon, it could add another shot every 3 rounds using the weapon, or whatever you can think of.
I am obviously bored.