Need to amend 2019 taxes online, it was for year 2018

".....somehow my 2018 taxes got filed as 2019."

That would be because you used the wrong software.  The online program is ONLY for 2019; it cannot be used for any other year, nor can you change the year when you use it.  

The only thing you can do to amend the return you messed up is to enter your 2019 income information instead of the 2018 income you put in.  That form can ONLY be used for 2019.

To file correctly for 2018:  (And....this will not be "amending."  You still need to prepare an original 2018 tax return using 2018 software and 2018 forms)

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.