Macros using selections are not a great idea and I haven't ever felt the need to use discontinuous selections for anything.
How about you specify the type of field you want to update. The sample document you posted had a bunch of pageref fields so you can update just those like this. It is simple to expand this macro to include or exclude other types of fields if you know what you want to do.
Code:
Sub UpdateFields()
Dim aFld As Field
For Each aFld In ActiveDocument.Fields
Debug.Print aFld.Type & ": " & aFld.Code
If aFld.Type = wdFieldPageRef Then
aFld.Update
End If
Next aFld
End Sub