Go to the following link for instructions to amend a 2018 tax return that you filed with TurboTax Online.
How do I amend my 2018 TurboTax Online return?
An amended return has to be filed on a special form, Form 1040X. It cannot be e-filed. You have to print it, sign it, and file it by mail. It takes the IRS up to 16 weeks to process an amended return.
