Building upgrades?

[Dimination]

How do I upgrade buildings? It says I need resource items, but I don't know what those are or how to get them. Any help would be awesome.

[some moron]

Does it really say resource items? or does it say tools cause those can be bought at your home city, though your ships might be automatically selling them faster than they can resupply. So buy some to your warehouse and manually use a ship to transfer them, not sell them.

[Dimination]

I eventually figured it out. It actually says resource items, but I tried buying the item that matched the picture, which was iron wares. Thanks for your help, though.