Dim wrdApp          As Object, wrdDoc As Object
On Error Resume Next
Set wrdApp = GetObject(Class:="Word.Application")

If wrdApp Is Nothing Then
    Err.Clear
    Set wrdApp = CreateObject(Class:="Word.Application")
    
    If wrdApp Is Nothing Then
        MsgBox "Microsoft Word could Not be found - Aborting"
        Exit Sub
    End If
End If
On Error GoTo 0
Application.ScreenUpdating = FALSE
Application.EnableEvents = FALSE
With wrdApp
    .Application.ScreenUpdating = FALSE
    Set wrdDoc = .Documents.Open("D: \08-02-21\test.docx")
    .Visible = TRUE
    .Activate
    
    Dim HdrText     As String
    Dim BoldRange   As Range
    Dim HdrRange    As Range
    
    'Set Variable equal to Header Range
    Set HdrRange = wrdDoc.Sections.Item(1).Headers(wdHeaderFooterPrimary).Range
    
    ‘Data To add To Header
    HdrText = ThisWorkbook.Sheets("details").Cells(4,2).Value
    
    'Add Text To Word Header
    HdrRange.Text = HdrText
    
    'Bold Only First Sentence in Header
    Set BoldRange = HdrRange.Words(1)        'Get First Word
    BoldRange.Expand (wdSentence)        'Expand To Entire Sentence
    BoldRange.Font.Bold = TRUE        'Bold Entire Sentence
End With