Where do inventory items go?

I accidentally put a tower or barricade (I can’t remember which) into “inventory”.  Where does that go?  Can I get it back?  Also, what is the best way to change a tower or obstruction to a more advanced one?

Click a random tile of the path where there aren’t towers or barricades placed, it will appear a menu -> click the center icon that is not greyed out: will appear the tower icon if you clicked on the grass otherwise will appear the trap icon if you clicked on a walking path. 

The tower icon menu is where all your towers are located and where you can also build new ones,

The trap icon menu is where all the barricades, blockades and traps are located and where you can build also here new ones.