Each weapon can be upgraded from the Armory using In-game currency. The price and type of currency required to upgrade each weapon to the next level can be found in the Armory when viewing each weapon.
Upgrading weapons can provide the weapon with different Perks and Abilities.

Perks provide the player a passive bonus when using the weapon during gameplay.
Abilities are special features of the weapon which can be activated by tapping their corresponding icon during gameplay. Some abilities require the player to shoot the weapon while others provide a temporary bonus for a limited duration. Once an ability has been used the player will have to wait through a cooldown process until the ability can be used again.

Descriptions of both the Perks and Abilities can be found by tapping and holding each icon on the Armory menu.