You need to build the Academy of Industry to upgrade factories. Tap the Academy to open a window displaying the list of factories you can improve. These are the attributes you can upgrade using ingots:
- The income you earn by selling factory items at the helicopter pad.
- The number of experience points you get for producing items.
- Production time for goods (it decreases).
- The number of shelves you have available for finished items (you can get an extra shelf).
You can't focus on upgrading just one attribute. Instead, you have to gradually improve all the different attributes to unlock the upgrades available at the Academy of Industry. Higher factory levels have more improvable attributes.
Different upgrade levels require different numbers of ingots, which are produced at the Foundry.