How to upgrade a building?

You want to upgrade your buildings to level 2 or more?

When you click on the building a menu appears! You can see three parts : the level, the extra payment of the building and the stocks.

On the level part you can see the next level and how many times you need to restock the building before upgrading it.

Once it is ready to be upgraded, an arrow appears and you can upgrade the building by crafting the objects needed.