Can I add a 1098 mortgage interest form after I've already filed?

If you haven't yet filed your return or if your return is rejected you can just add the mortgage interest to your return and file.

If you have filed your return and it's been accepted, you can amend your 2016 return by following these steps:

1.      Sign back in to TurboTax and go to your Tax Timeline.  [See Screenshot #1, below.]

2.      In the section Some Things You Can Do (below the grey line), click on Amend (change) return. [Screenshot #2]

3.      On the screen, Are you sure you'd like to amend, click the box Amend using TurboTax Online.

4.      On the screen, Have you already filed this return, click the Yes button and then click the button next to I need to amend my 2016 return and click Continue.  [Screenshot #3 ]

5.      On the screen, What do you need to change on your return, scroll down and click on the Update box next to the type of item you need to change -- in your case, Deductions & Credits. [Screenshot #4]

6.      Continue through the screens, entering the requested information.

You can track your amended taxes with the IRS tracking tool. Your amended return will take up to 3 weeks after you mailed it to show up in the IRS system.  It takes the IRS around 16 weeks to process your amended taxes.

The processing time for amended state return differs from state to state. Contact your state to find out how long it takes them to process your amended taxes.