If you have already filed, here are the steps:
Wait until your return is either rejected or approved.
If it's rejected, you can return to the program to add the new information while fixing the error.
If your return is approved, you would need to amend if adding the new information changes your original refund amount or tax due. To amend, please click here 2018 Amend. Follow the instructions in the program and it will tell you if the correction changes the result. If you do need to amend, please wait until your return is fully processed and you have received your refund or your payment has cleared.
You can file a previous tax return based on prior income if you have not filed a tax return for that year, if you did file a tax return then you will need to file an amended return.
Here is useful information for filing an amended return:
https://turbotax.intuit.com/tax-tips/amend-return/how-to-correct-federal-tax-returns/L67t0F7il
Please click this link to learn how to file a prior year tax return with TurboTax, you will need to mail it in as the IRS does not accept e-filed tax returns for prior years: