How do you upgrade buildings in kknd2?

In KKND2 (Krush, Kill 'n' Destroy 2), you upgrade buildings by gathering resources and then selecting the building you want to upgrade. Click on the building, and if an upgrade is available, an option will appear in the interface. Ensure you have the necessary resources and meet any prerequisites for the upgrade, then confirm the action to enhance the building's capabilities. Upgrades typically improve production rates, health, or unlock new units.