If the 2018 tax return has already been filed and accepted then you will need to amend the return to add the additional income to the tax return. Before amending the return wait for the tax refund to be received or if taxes were owed and paid, wait for the payment to be processed.
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
