// enter your customized code here
MercatorUi.Forms.Billing.BillingForm billingForm = (MercatorUi.Forms.Billing.BillingForm)clickedButton.Form;
billingForm.BillingEngine.LignesVRecords.Where(l => l.TAUX_TVA.CompareTo(21, 2) == 0).ToList().ForEach(l => l.TAUX_TVA = 6);
billingForm.BillingEngine.UpdateAmounts();