If the 2018 federal tax return was filed and accepted by the IRS, then you will have to amend the tax return to make any changes to the filed return.
See this TurboTax support FAQ for amending a 2018 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27579
