Public Function upd()
Dim rst As DAO.Recordset
Dim rst1 As DAO.Recordset
Set rst = CurrentDb.OpenRecordset("select * from Tabl2")
rst.MoveFirst
Do While Not rst.EOF
    Set rst1 = CurrentDb.OpenRecordset("select D from Tabl1 where A=" & rst.Fields(0) & " and B=" & rst.Fields(1) & " and C=" & rst.Fields(2))
    If rst1.EOF Or rst1.BOF Then GoTo 1
    rst.Edit
    rst.Fields("D") = rst1.Fields(0)
    rst.Update
1:
    rst1.Close
    Set rst1 = Nothing
    rst.MoveNext
Loop
rst.Close
Set rst = Nothing
End Function