How do you upgrade buildings in Settlers 7 : Paths to a Kingdom?

I just got this game and I have played the first two missions of the campaign, but I didn't find out how to upgrade buildings. When I click one it says Upgrade 0/2..... but there is no button for upgrading... there are only the buttons for demolishing the building or constructing something on one of its slots. Please does anybody know how to upgrade the buildings ??? Best Answer 10 points !!!