If you completed and filed a 2018 tax return using the online editions, have received the tax refund or paid the taxes owed then you can start an amended tax return. An amended return can only be printed, signed, dated and mailed by you, it cannot be e-filed.
See this TurboTax support FAQ for amending a 2018 tax return originally completed and filed using the online editions - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/01/27577
