Do you mean that you want to amend your 2019 tax return, that you filed in 2020? You will file your 2020 tax return after the end of 2020. Go to the following link for instructions to amend a 2019 tax return that you filed with TurboTax Online.
An amended return has to be filed on a special form, Form 1040-X. It cannot be e-filed. You have to print it, sign it, and file it by mail. It normally takes the IRS up to 16 weeks to process an amended return. However, the IRS is currently not processing mailed tax returns because of the Covid-19 pandemic. When they start to process them again they will have reduced staff and a big backlog, so it will be a long time before your amended return is processed.
Don't file the amended return until your first return has been accepted and you have received the original refund.
If you will owe additional tax, or will have to pay back part of your refund, you should file the amended return before July 15, 2020 to avoid possible penalties and interest.