After you file

STOP  You have just used the wrong software.  Do NOT file that return if you used the online program.  The online software is ONLY for a 2019 return.  You cannot change it to 2018 or any other year.   Sorry---but you have to clear the 2018 information out.  If you are going to prepare a 2019 return you can use the account for 2019.  Otherwise, clear it or abandon it.

If you have not paid or registered, you can clear it.

CLEAR AND START OVER

https://ttlc.intuit.com/questions/2586254-how-do-i-clear-and-start-over

And if you think this is more work--it is -- but the months you would wait for amending an incorrect return would be much worse.  

To file your 2018 return correctly:

Online preparation and e-filing for 2016, 2017 and 2018 is permanently closed.

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)

https://turbotax.intuit.com/personal-taxes/past-years-products/

You may also want to explore purchasing the software from various retailers such as Amazon, Costco, Best Buy, Walmart, Sam’s, etc.

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.