To flip buildings, first Click on the red Toolbox Icon. This icon is in third from the left in your row of circles at the bottom of your game screen.
Once you have clicked it, you will see a whole row of other tools! The one you're looking for is the second from the left, right next to the move arrows! It looks a little bit like a turning symbol. Click on that symbol, and then click the building you want to flip, and voila!