I followed this guidance from TurboTax boards and used EasyGuidein my 2019 filing process to tell IRS that my basis should have only been 5500 in previous year filing:
You have two choices: Either correct the amount in 2018 TurboTax and provide explanation that your 2013 Form 8606 incorrectly had $18,019 on line 2 instead of the $34,950 from line 14 of your 2012 Form 1099-R, that no distributions or conversions occurred in 2013 through 2017 and that you are correcting that by adding an adjustment of $16,931 on line 2 of your 2018 Form 8606, or file amended 2013 through 2017 Forms 8606.
Amending each of the incorrect Forms 8606 is probably more proper since it will make it explicit that none of your basis was applied to any distribution or Roth conversion. The IRS might charge a $50 fee for each of the amended forms (this probably covers the cost of verifying that these have no effect on your tax liability for 2013 through 2017); they'll bill if they do. However, just making the change on your 2018 tax return with explanation will probably raise less attention and might not get reviewed at all.
In the case where you just modify the value in 2018 TurboTax, if you use the EasyGuide button just mark the first two boxes on the Any of These Apply? page. TurboTax will prompt for explanation whether or not you use the EasyGuide button.
In the case where you amend the 2013 through 2017 Forms 8606, to avoid including any explanation with your 2018 tax return you'll need to prepare your amended the 2017 Form 8606 with 2017 TurboTax, then you'll need to clear your 2018 tax return and start over to transfer in from the amended 2017 tax file. If you just modify the value in 2018 TurboTax without transferring in the correct amount from the amended 2017 tax file, TurboTax will still prompt you for explanation.
