I need to back out of this app because this is my 2018 info.. How do I do that?

You said "app."   Are you using the mobile app, and did you mistakenly try to prepare a 2018 return?    At this point in time, the mobile app and Online TurboTax are only for a 2019 return. 

If you entered 2018 information by mistake into the 2019 mobile app, you can clear it out and start over.  And then you can prepare the 2019 return in the app.  I'll tell you how to prepare the 2018 return lower down in this posting.

FAQ:  How do I clear and start over in the mobile app?

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-clear-and-start-over-in-the-mobile-app/00/26808

For a 2018 return, if you want to use TurboTax, you'll need to use 2018 desktop software (CD/download) installed on your computer, and it cannot be efiled.   I'll also tell you another option at the end of this post if you want to prepare it manually with forms and instructions from the IRS.

You can purchase 2018 desktop software as a download at this page and install it on your computer:
https://turbotax.intuit.com/personal-taxes/past-years-products/

Minimum system requirements for Windows version:
https://ttlc.intuit.com/questions/1901571-minimum-system-requirements-for-turbotax-windows-software

Minimum system requirements for Mac version:
https://ttlc.intuit.com/questions/1899173-minimum-system-requirements-for-turbotax-mac-software

The 2018 return cannot be efiled.  It will need to be printed, signed, dated, and mailed.  It's a good idea to mail by a method that has tracking, such as certified mail, priority mail, etc.

If you have both Federal and state returns, you need to mail each one separately to its respective tax agency. 

If you are due a Federal refund, it takes about 6 weeks for the IRS to process a mailed return and deliver the refund.

OR another option is to download the 2018 forms and instructions for free from the IRS website and fill them out manually.

https://www.irs.gov/forms-pubs/prior-year