Upgrading improves the efficiency of the infrastructure in your growing empire. The resource used to upgrade is Oil. To upgrade, pay the cost, pick up a technology cube from any green box on your Player Mat, and place it on any empty red box with bracketed borders. Red boxes with solid black borders are off limits for upgrades.
You can choose any top row cube and move it to any bottom row space available. They don't have to be in the same column.