See this TurboTax support FAQ for amending a prior year tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-a...
Go to this IRS website for the Form 1040-X - https://www.irs.gov/pub/irs-pdf/f1040x.pdf
Go to this IRS website for the Form 1040-X instructions - https://www.irs.gov/pub/irs-pdf/i1040x.pdf
