Once a tax return has been filed, the only way IRS will let you change/add information is by filing an amended return (1040X). You can do this in TurboTax, but you'll need to wait until IRS has processed your original return and issued any refund. Then you can prepare the amended return and make the necessary changes.
IRS only accepts the amended return on paper, so you'll have to print, sign, and mail it.
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...
