How to demolish buildings in the game?

If you mean buildings in your city, you cannot demolish them. you can tap and hold to move them around though. Wall and roads can be removed and replaced.

If you mean enemy buildings in Reign of chaos, then these can be attacked during wars and demolished. You need to have an adjacent alliance tile to any building you want to attack then you will have the option to do so. Each building will have a durability and you need to keep attacking until the durability reaches 0.