Sub Macro1()
Dim oRng As Range
Dim oCC As ContentControl
Dim lngCC As Long
Dim iLst As Integer
Dim strListItem As String
Const strList As String = "C:\path\ccList.txt"
Dim iFile As Integer: iFile = FreeFile
    Set oRng = Selection.Range
    For lngCC = 1 To oRng.ContentControls.Count
        Set oCC = oRng.ContentControls(lngCC)
        If oCC.Type = wdContentControlComboBox Or _
           oCC.Type = wdContentControlDropdownList Then
            'leave the prompt text and delete the rest
            For iLst = oCC.DropdownListEntries.Count To 2 Step -1
                oCC.DropdownListEntries(iLst).Delete
            Next iLst
        End If
        'add the new list from a text file
        Open strList For Input As #iFile
        Do Until EOF(1)
            Line Input #1, strListItem
            oCC.DropdownListEntries.Add strListItem
        Loop
        Close #iFile
    Next lngCC
lbl_Exit:
    Set oRng = Nothing
    Set oCC = Nothing
    Exit Sub
End Sub