All you have to do is go to a gun vending machine icon on the map, or in an outpost. When you are there you can buy upgrades for some of your guns. Radio towers get you free guns, so I would suggest getting a few radio towers to then be able to buy weapon upgrades for your gun. If you are strapped for cash, I suggest getting upgrades to sights, such as night sights and red dot sight, as they are the cheapest upgrades. You can buy upgrades for the same weapons and still count for the trophy, and skins do not count when it comes to the trophy. This also counts towards the "Aftermarket Junkie" achievement, where you buy all upgrades and skins for one gun.