From the manual, page 49:
Undoing operations (undo) This unit allows you to undo, for example, unwanted  operations and recordings that were unsuccessful.  Undo can be used on many ordinary operations,  including editing and recording operations. This  unit has two types of undo functions. Single undo  can be used with a simple button press, while multiundo allows you to return to and undo a specific past  operation.
NOTE The UN/REDO [HISTORY] indicator lights when a  single undo or multi-undo has been conducted.
Undoing the last operation (single  undo) Press the UN/REDO [HISTORY] button to return to  the state before the last operation was conducted.  After doing this, the UN/REDO [HISTORY] indicator  lights, showing that redo (see below) is possible.
UN/REDO [HISTORY] indicator 88 Cancelling an undo operation (redo) Press the UN/REDO [HISTORY] button when it is lit  to return to the state of the last operation. The UN/
REDO [HISTORY] indicator turns OFF.
NOTE If single undo is used when multi-undo (described  below) has already been used to return the  unit to a previous state, it will go one more step  backward. If you press the UN/REDO [HISTORY]  button again to redo the step, the single undo  operation is cancelled, and the previous multiundo state is restored. In this case, the unit is not  restored to the most recently conducted operation,  so the UN/REDO [HISTORY] indicator remains lit  because further redo operations are possible.
Undoing a previous operation (multiundo) 1.    Press and hold the UN/REDO [HISTORY] button  to open the HISTORY screen.
    The HISTORY screen shows the operation history  as a list.
    Each operation in the list is called an “event” .
    The event list starts at the bottom with event  number 0 (-START UP-).
    The most recent event has an asterisk (*) to its  right.
    All events that occurred after a selected event  can be undone at once. This operation is called  “multi-undo” because it allows you to undo  multiple events and return to an earlier state.
2.    Use the DATA wheel to select the event that you  want to return to (all events after the selected  event will be undone).
3.    Press the F2 [EXEC] button to execute the multiundo and restore the state just after the selected  event was conducted.      In this state, the UN/REDO [HISTORY] indicator  lights.      The unit has been returned to a previous state,  but the event information undone through the  above operation is not erased. On the HISTORY  screen, a * appears to the right of current event  after undoing.
    If you make a new recording or new edits  after using multi-undo (UN/REDO [HISTORY]  indicator lit), the event history after the current  event is cleared and the last operation conducted  becomes the most recent event.
NOTE When the HISTORY screen is open, the UN/REDO  [HISTORY] button does not function, so single  undo and redo cannot be used.
Pressing UNDO performs a single operation UNDO/REDO.  Press and hold opens the history menu.