Sub aaa()
  Dim oSec As Section, oHead As HeaderFooter
  Dim oTbl As Table, oCell As Cell
  For Each oSec In ActiveDocument.Sections
    For Each oHead In oSec.Headers
      For Each oTbl In oHead.Range.Tables
        With oTbl
          'this would be the preferred method
          .RightPadding = 0
          .LeftPadding = 0
          .TopPadding = 0
          .BottomPadding = 0
          .Spacing = 0
          'and include 'same as table' for all cells
          'But there isn't a method to make all cells 'same as table' other than with SendKeys
          '... instead we will assign the same settings to every cell
          For Each oCell In oTbl.Range.Cells
            With oCell
              .TopPadding = 0
              .BottomPadding = 0
              .LeftPadding = 0
              .RightPadding = 0
            End With
          Next oCell
        End With
      Next oTbl
    Next oHead
  Next oSec
End Sub