27 C.F.R. § 70.127
Overpayment of installment
If any installment of tax is overpaid, the overpayment shall first be applied against any outstanding installments of such tax. If the overpayment exceeds the correct amount of tax due, the overpayment shall be credited or refunded as provided in §§ 70.122 to 70.124 of this part, inclusive.