If your original tax return that was mailed to the IRS was sent back because it was unsigned, you just sign and date the return and mail it back to the IRS. An amended return is only used if the original return was accepted by the IRS and the original return has to be changed. Your original return was never accepted by the IRS since it was unsigned.
If your return has a tax refund there is no penalty for filing a return with a tax refund after the due date.
If your return has taxes owed, then there will be a late payment penalty along with interest on the unpaid taxes.
