My parents didn't claim me and i need to claim myself as a dependent to get my stimulus check. ive already got my tax return and dont know how to change it. what do i do?

the tax law isn't whether they claim you but could they claim you.   The IRS is perfectly willing to let parents not claim they're qualifying dependents. the IRS is not willing to let those dependents claim themselves.

you could be claimed as a qualifying child if all these tests are met
• you have the same principal abode as the other party for more than ½ the tax year. Temporary absences like for school are ignored
• If not a full-time student, under 19 at the end of the tax year. If a full-time student under 24 at the end of the tax year.
• you haven't provided over ½ your own support
• you didn't file a joint return unless there was no tax liability but merely filing joint to facilitate refund of taxes withheld or estimates paid

or as you could be a qualifying relative if all these tests are met
> you're related or if not you lived with the other party for the entire year
• your gross income for 2019 less than $4,200
• the other party provided over ½  your support
• you arent a qualifying child of another taxpayer

what will the IRS do if you don't mark the box so you get the stimulus,  don't know?