The TurboTax online editions are Only for tax year 2019. You will be asked to provide the 2018 AGI to e-file the 2019 tax return. If you did not file a 2018 tax return then enter a 0 (zero) for the 2018 AGI.
What? What are you trying to do? The online software program is ONLY for 2019. It cannot be used for any other tax year. If you are trying to use it for any other year, you are making a big mistake. You cannot change it to a different year.
If you are trying to use desktop software that says 2018, you cannot update that or change it to 2019. Each tax year has its own specific software program. You need 2019 software for a 2019 return.
To file a return for a prior tax year
If you need to prepare a return for 2016, 2017, or 2018 you can purchase and download desktop software to do it, then print, sign, and mail the return(s)
Remember to prepare your state return as well—if you live in a state that has a state income tax.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.
Note: The desktop software you need to prepare the prior year return must be installed/downloaded to a full PC or Mac. It cannot be used on a mobile device.
If you are getting a refund, there is not a penalty for filing past the deadline. If you owe taxes, the interest/penalties will be calculated by the IRS based on how much you owe and when they receive your return and payment. The IRS will bill you for this; it will not be calculated by TurboTax.