It's better to read the manual, but here are some tips.
Enter/exit the remapping mode: Progrm + Remap (F12).
Drop the layout: Progrm + Shift + Disable Macro (F10).
When remapping, make changes against the default layout, not the current one. It's the default that becomes available when you enter the remapping mode.
To swap two keys: while in the remapping mode, press key1 and key2, then key2 and key1, then exit.
To get back the "lost" key just map it again assuming the default layout (see above). Note that you probably changed some key caps, so it's better to rely on the file with the default layout instead.