Yes, you can replace a faulty battery, do not forget to disconnect wall power first, otherwise you’ll fry balancing resistors and possibly transistors.
Re battery order, I think this info is in the user manual.
The leftmost group of four icons represent the battery pack on the left side of the device, and the top icon in each group represents the leftmost cell in each pack—assuming you look at MNT Reform when flipped on its back and the battery closer towards you.