Function UpdateHeader(oDoc As Document) As Boolean
Dim oSection As Section
Dim oHeader As HeaderFooter
Dim oTable As Table
Dim oCell As Range
    On Error GoTo err_Handler
    For Each oSection In oDoc.Sections
        For Each oHeader In oSection.Headers
            If oHeader.Exists = True Then
                If oHeader.Range.Tables.Count = 1 Then
                    Set oTable = oHeader.Range.Tables(1)
                    If oTable.Rows.Count = 2 And oTable.Columns.Count = 2 Then
                        Set oCell = oTable.Cell(1, 1).Range
                        oCell.End = oCell.End - 1
                        oCell.Text = "The new text for the cell"
                    End If
                End If
            End If
        Next oHeader
    Next oSection
    UpdateHeader = True
lbl_Exit:
    Set oSection = Nothing
    Set oHeader = Nothing
    Set oTable = Nothing
    Set oCell = Nothing
    Exit Function
err_Handler:
    UpdateHeader = False
    GoTo lbl_Exit
End Function