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. See this for whether someone can claim you or not: https://blog.turbotax.intuit.com/tax-deductions-and-credits-2/family/who-can-i-claim-as-a-dependent-...
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
You should use TurboTax to prepare the amended return. Page 1 of form 1040X needs to show that nothing changed on most of the lines but that you did receive the refund from the original return. On page 2 you need to enter an explanation for why you are amending the return. You could say something like: "I am amending my return because I mistakenly chose that I could be claimed by someone else on my original return and I do not meet the requirements to be claimed by someone else."
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.
