On this page of the guide to Aliens: Fireteam Elite you'll learn more about improving your character by configuring your weapon and modify it with various attachments. You'll learn just how important it is to often modify your weapon.
As you progress through missions you'll obtain constantly increasing amounts of credits and items, including weapon attachments you can upgrade your weapon with. You can do the same thing by visiting a trader and buying weapon attachments. They are very useful and worth investing into, as they increase your weapon's power.
To modify a weapon, select it in the weapons tab or your character's menu, then select mods. From there you can choose three weapon modifications, usually the barrel or the magazine. Every weapon attachment increases a specific stat. Some of them have unique effects, so it's worthwhile to read their descriptions and choose the best one.