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. It's taking much longer now.
If you file an amended return to claim yourself, you parents must also file an amended return to take you off their tax return. They should have received a $500 other dependent credit that they will have to pay back.
If they don't amend their return, the IRS will come to both you and your parents later in the year asking for documents to support your claims about your dependency.
An alternative for you is to wait until you file you 2020 tax return and not check the box that you can be claimed as a dependent 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.
