Sub UpdateDocumentHeaders()
Application.ScreenUpdating = False
Dim strFolder As String, strFile As String, StrTxt As String
Dim wdDocTgt As Document, wdDocSrc As Document
strFolder = GetFolder
If strFolder = "" Then Exit Sub
StrTxt = InputBox("Text to add to headers? e.g. ""March 1, 2018""")
If Trim(StrTxt) = "" Then Exit Sub
Set wdDocSrc = ActiveDocument
strFile = Dir(strFolder & "\*.doc", vbNormal)
While strFile <> ""
    If strFolder & "\" & strFile <> wdDocSrc.FullName Then
        Set wdDocTgt = Documents.Open(FileName:=strFolder & "\" & strFile, _
        AddToRecentFiles:=False, Visible:=False)
        With wdDocTgt
            .Sections(1).Headers(wdHeaderFooterPrimary).Range.InsertAfter StrTxt
            .Close SaveChanges:=True
        End With
    End If
    strFile = Dir()
Wend
Set wdDocSrc = Nothing: Set wdDocTgt = Nothing
Application.ScreenUpdating = True
End Sub

Function GetFolder() As String
Dim oFolder As Object
GetFolder = ""
Set oFolder = CreateObject("Shell.Application").BrowseForFolder(0, "Choose a folder", 0)
If (Not oFolder Is Nothing) Then GetFolder = oFolder.Items.Item.Path
Set oFolder = Nothing
End Function