See this TurboTax support FAQ for amending a 2019 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/01/27577
Or go to this IRS website for a federal amended tax return, Form 1040-X - https://www.irs.gov/forms-pubs/about-form-1040x
