This will come later on in chapter 3 or 4 depending on how much money you have, simply just keep upgrading an attribute of your weapon until you can’t raise it no more. Some go to level 6 some go to level 7, the attribute I upgraded was accuracy as it only unlocks at level 6. You can’t do this any earlier due to certain levels being locked until you progress deeper into the story.
A nice way to get credits early on is at the end of the 2nd Chapter when you are in the sewage plant after saving your comrades, you wait for an elevator and Crazy monkey bots, the small brown ones will infinite spawn and will keep coming to you. Just walk up to them and shoot/melee, they are one hit with melee. Make sure not to get on the elevator.