I selected the wrong form to start my return. How do I go back and start with correct form?

You must clear and start over to downgrade to a lower version. You should be able to do that if you have not paid or registered. Here is a link to help you: https://ttlc.intuit.com/questions/2586254-using-clear-start-over-in-turbotax-online

You are not using the wrong form.  Everyone's tax return goes on a Form 1040.  But if you are not self-employed you want a less expensive version of the software.  You can start over in a less expensive version.  Or perhaps you qualify to use a free verison.

CLEAR AND START OVER

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

WHY DO I HAVE TO PAY?  I WANT THE FREE EDITION

The information that you can enter in Free Edition is pretty limited now.  Thanks to the new tax laws that began for 2018 returns, there are no more simple Form 1040EZ or 1040A's.  Everything goes on a Form 1040 that has three extra "schedules" with it, and if you need any of those schedules, you are not able to use the Free Edition.  Using the standard deduction instead of itemizing does NOT mean you will not need any of those schedules.https://ttlc.intuit.com/questions/4511011-what-happened-to-the-1040a-and-1040ez

https://www.irs.gov/forms-pubs/about-form-1040 

If you qualify to use it, there is another full-featured free version of the software:

Try Free File: 

You qualify if your income was $36,000 or less, or $69,000 or less if active duty military, or if you qualify for Earned Income Credit

https://ttlc.intuit.com/questions/1900583-what-is-turbotax-free-file-program

• https://ttlc.intuit.com/questions/2026912-how-do-i-switch-to-turbotax-free-file-program

Note:  If you do not qualify to use Free File then you will have to upgrade to Deluxe or higher in the paid versions of the software.   

Online Deluxe $60  State software $50

Or—-Use this IRS site for other ways to file for free

https://apps.irs.gov/app/freeFile/