Contraption upgrades are done through little side missions you can go through on Eva's Hammer after you've acquired each contraption. You can get them all their respective upgrades in a single play-through.
Gamerevolution.com 's guide over which contraption is right for you also details where they all are and where their upgrades are. Be sure to check them out!
https://www.gamerevolution.com/guides/353631-wolfenstein-2-c...