There's no charge to file an amended return (1040X).
You'll have to file it on paper (print, sign, and mail) since IRS won't accept e-filed amended returns. Here's a link to the TurboTax support page for amending returns, which will get you started: https://turbotax.intuit.com/tax-tools/tax-tips/IRS-Tax-Return/Amending-Your-Income-Tax-Return/INF120...
