My 2019 tax return was accepted, I qualify for the stimulus check. When I went back and checked, I accidentally marked I was a dependent, and I am not. What do I do now?

[Texas Roger]

You are not eligible for the stimulus payment if you checked the box on your tax return that you can be claimed by someone else.  You must check the box if you meet the requirements to be claimed by someone else even if they don't claim you.

If you do not meet the requirements to be claimed by someone else, you can prepare, print, and mail an amended return form 1040X to "uncheck" the box. DO NOT uncheck the box until you start the amended return. Here is a link to help you: https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

Amended returns must be printed and mailed. They can't be efiled. You must print and mail it yourself. In normal times, it takes the IRS about 16 weeks to process an amended return.

An alternative for you is to wait until you file you 2020 tax return and not check the box on that return. The stimulus payment is actually an advance on a credit claimed on your 2020 tax return. You could get it when you file your 2020 return. 

[MinhT1]

As you indicated that you can be claimed as a dependent, you are not eligible for the stimulus payment.

The real question is whether you can be claimed as a dependent (see this TurboTax article). If you can be claimed as a dependent, but no one actually claims you, you are still ineligible for the stimulus payment.

If no one can claim you as a dependent, you'll need to amend your 2019 tax return in order to be eligible for the stimulus payment.

To amend your return, please follow the instructions in this TurboTax Help topic.