If on PC in Inventory, you have to click on the hand-holding-hammer icon* below the selection kit. A window will pop up showing your options. Note that if you choose an "Upgrade" and you don't have any buildings eligible for that upgrade, you'll get a message telling you that and you won't be able to use that. You should still be able to use other options from that selection kit, however, such as placing a level one of that building.
*On mobile the icon is simply a hammer, but the rest is the same.