Any refund for the vehicles that was either sold/ destroyed or stolen, used less than the mileage use limit or tax overpaid can be claimed using Form 8849 with duly filled return. Schedule 6 is used for claims not reportable on Schedules 1, 2, 3, 5, and 8, including refunds of excise taxes reported on:
https://www.irs.gov/pub/irs-dft/i8849s6--dft.pdf is the link in which one can find the instructions for Form 8849-Schedule 6 – Other Claims. As per the instruction in the IRS website:
A claim for refund may be made for the following:
An overpayment of tax due to a mistake in tax liability previously reported on Form 2290. The claim is made by the person that paid the tax to the government.
A pro rata refund of the tax paid on Form 2290 for a vehicle that was sold, destroyed, or stolen before June 1 of any period (July 1 – June 30) and subsequently not used during the period. The claim is made by the person in whose name the vehicle is registered at the time it was sold, destroyed, or stolen. To make a claim for a vehicle that was sold, destroyed, or stolen, the following information must be attached to Schedule 6.
Any vehicle on which the tax was paid on Form 2290 if the vehicle was used 5,000 miles or less on public highways (7,500 or less for agricultural vehicles) during the period (July 1 – June 30). The mileage limitation applies to the total mileage a vehicle is driven during a period regardless of the number of owners of the vehicle. A claim for this mileage refund cannot be filed until after June 30 of the period. The claim is made by the person that paid the tax to the government.https://www.irs.gov/pub/irs-access/f8849s6_accessible.pdf is the link in which one can find the Form 8849-Schedule 6 – Other Claims.