You can use the form's RecordsetClone to navigate to any record in the
form's recordset. For example:
With Me.RecordsetClone
If Me.NewRecord then
.MoveLast
Else
.BookMark = Me.Bookmark
.MovePrevious
If .BOF then .MoveFirst
End If
' your RecordsetClone is now on:
' the last record if you are on a new record
' the first record if you are on the first record
' otherwise, the previous record
Me![SomeField] = ![SomeField]
End With