Moves the range or selection to the previous subdocument. If there isn't another subdocument, an error occurs.
expression.PreviousSubdocument
expression Required. An expression that returns a Range or Selection object.
Example
This example switches the active document to master document view and selects the previous subdocument.
If ActiveDocument.Subdocuments.Count >= 1 Then
ActiveDocument.ActiveWindow.View.Type = wdMasterView
Selection.EndKey Unit:=wdStory, Extend:=wdMove
Selection.PreviousSubdocument
End If