How can I boost weapons?

You can boost weapons by choosing to spend weapon parts and metal in the workshop screen. To get there, select your hangar, then the workshop, then the weapon you wish to improve. Next tap on Boost and select the amount of resources you wish to invest.

Weapon parts can be earned by scrapping unused weapons in the workshop, by completing missions or directly obtained from crates, while metal can be earned by playing missions, completing objectives, or also by opening weapon crates.