Buildings can be upgraded by using materials collected in scenes or as rewards when finishing scenes.

Simply click or tap on the building to start the upgrade. A pop-up will appear and it will show you how many stars are needed until your building is fully upgraded.
There are two ways to upgrade a building:

1. Click on "Buy All for xxx" using Greens. You may upgrade the entire building in one go, without needing to collect materials.
2. Click on "Buy for" under each material. This allows you to upgrade the materials separately, one by one.  

All buildings have 3 levels of upgrade but construction times vary from building to building.