After completing the quest “into the storm” you will be tasked with talking to a guy named Alvin.
Speak to him and he will take you though the process of how to upgrade your weapons.
Once the dialog is done open your inventory, select a plasma weapon, pick three chips to use that you have collected from fallen terminators and other enemies in the world and put them together.
Trick to this is that the chips have to have matching patterns.
An example is if you start with a pattern that is a square on one end the pattern on the last chip as to be a square, you will also have to match the patterns in the middle but the two outer patterns must match.