First do NOT start the amendment process until the original return has been fully processed and you have the original refund in hand or you have paid the original balance due.
See this TurboTax support FAQ for how to amend a tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-...
