How do I destroy a tower to replace it with a better one?

I have a tower, I sell it to put in place of it a better tower, how do I destroy it and verify that the site occupied this tower is free.

Why not have all tower in a single sprite Frames or animation and have if sell change the frame or animation to the empty slot. and have a variable that tell which tower it is. Instead of creating mulitple tower just have it all in one sprite and place it on the board.

It makes sense what you said, I will try to do here.