The Weapon Upgrade Kit fabrication plan can be found in the Crew Quarters, but you need a few upgrades in order to get it. The room it is found in belongs to Sarah Elazar ,located in Crew Cabins A. There is a shaft next to the door that is blocked by a large box. You have the option to use "MORPH - Mimic Matter 1" or "ENGINEER - Leverage 3" to get into the room. The workstation inside has the fabrication plan for the Weapon Upgrade Kit. You gain access to the workstation with the "Hacking 2" ability.