To delete an existing hill, enter edit mode by pressing the Edit button (left end of the top navigation bar). Then tap (once) the hill you wish to delete. The outer contour of the selected hill will now be decorated with eight little dots to indicate that the hill has been selected, and the button with the trash can icon (bottom tool bar, 3rd. button from right) will become enabled. Pressing this trash button will delete the selected hill from the current configuration.
Note that after you pressed the Edit button, the button turned into a Done button to indicate that you are now in editing mode. You must leave editing mode before you will be able to make the robot walk around on the landscape you’ve just created.