How do I upgrade my walls and floor on Last Day on Earth?

The zombie hordes keep destroying my walls, how am I supposed to upgrade them?

Zombie hordes frequently come through your base on Last Day on Earth, destroying your walls as they do. Upgrading walls from wood to better wood, to stone and so on helps keep them at bay. You also need better floors to hold certain items. It isn't immediately obvious how to upgrade your walls and floor though. Tap on the saw&hammer icon as if you were going to build a wall/floor/etc., then tap on the wall or floor you want to upgrade to "select" it. Once it is selected a few options will pop up - a red brick wall with an "x" on it (tapping this will destroy your wall/floor) and an up arrow with the item requirements next to it. If you have the right ingredients (planks, stone, bricks, etc.), tap on the arrow to upgrade your wall or floor.

How to upgrade walls / floors on Last Day on Earth for iOS on iPhone and iPad.