You will have to download the tax data file ending in .tax2018 from online. And install the 2018 Desktop program to open it. If you paid for 2018 you can ask for the Desktop download. You need a full Windows or Mac computer to install it.
To get your tax file, Use this article for amending. It is for 2017 but will be the same for 2018. Just change all the dates to 2018 like .tax2018
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27371
After you get the program installed the first thing to do before you open your tax return .tax file is to update the program and install any state programs you had. Then open your file. So you first might need to start a fake return to be able to download the state program (go to FILE - NEW).
Contact Customer Service to get the program
5am-5pm Pacific Time Monday -Friday
https://support.turbotax.intuit.com/contact/
