Undo will reverse the last task your gadget did, whether that means removing the last set of blocks or built, or swapping the last set of blocks back.
If exchanging back, make sure you still have the necessary items in your inventory!
Undo remembers the last 10 actions the gadget made.
Undo can be accessed via HotKey or from the Settings Menu of your gadget.
The cut and paste gadget does not have undo functionality!