What is Depot, and how I can upgrade it?

When the game talks about the "Depot", it means your storage. Just upgrade the storage building for the specific resource you want to hold more of. You can upgrade it by tapping on the resource building you wish to upgrade, tap on the green arrow, and and two the upgrade button. If it doesn't work then you might not yet reach the requirements you need to upgrade the building. He requirements are shown above the upgrade button.