I'm doing 2020 taxes and discovered an error in my nondeductible IRA basis on Form 8606 from 2012 where the basis was incorrect. Do I need to amend or just update in 2020

[clarkh1]

The basis in 2012 was off by one year of contributions because we recharacterized 2010 contributions from Roth IRA to Traditional IRA when AGI was too high to contribute to Roth.  Somehow, the 2010 contributions were not entered into the basis back then and have been missed ever since until I realized it this year.  Wondering if we need to formally amend the 2012 return or just correct for 2020 going forward.  The difference is just under $2,000, so its not a ridiculous amount of money.

[KathrynG3]

The formal way to correct Form 8606 would require amending the return. However, it is a closed tax year. Since there is not a tax effect, you can capture these details in the Form 8606 interview. 

To do this in TurboTax, in the search box, type this exact phrase, including the comma: 8606, nondeductible ira contributions

This will bring you to the correct location. Continuing through the interview will lease to Any Roth IRA Conversions? and next: Tell Us the Years. 

Select 2015 and prior years and Continue.

For more details, see: Where do I find Form 8606?