Visit https://apps.irs.gov/app/picklist/list/formsPublications.html to download IRS forms. Or, you can visit a local IRS office.

Visit https://www.irs.gov/newsroom/tips-for-taxpayers-who-have-to-amend-a-tax-return on instructions on how to amend a return.

In particular, you need the following form: https://www.irs.gov/forms-pubs/about-form-1040x

You can prepare your amended return with TurboTax, and TurboTax will print the necessary forms. But an amended return cannot be e-filed. You have to print it, sign it, and file it by mail.

Go to the following link for instructions to amend a tax return that you filed with TurboTax.

https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed