What you are looking for should be under sponsor settings. Look for sponsor settings at the bottom right side of the starbase screen. There you should have many options for managing starbase upgrades by constructors.
Things are different in Crusade. You do upgrades by spending money and resources. The constructor is only needed to build the initial starbase.
In Crusade, you can customize Constructors to have longer range, faster engines, etc.
For the base game, you can additionally customize them to have more modules (which isn't needed in Crusade).