There is no undo button. 

Type cleargraphics or cg in the Command Centre to remove any stamped images and any turtle lines visible in the Work Area.

You can also see  the section on how to delete any extra turtles you do not want.

More info:  this is a coding language—not a 'ready-made' app—so it is important to start writing procedures early in your coding adventures, so that when 'bugs & errors" happen—and they should and they will!--you can adapt quickly.