I accidentally clicked that “Someone Can Claim Me As A Dependent”

I’m 22 years old, working and going to school full time. I clicked the box that said I could be claimed but I will not be and no one will be claiming me. How can I fix this? I do believe making this mistake has made my return 0$, and I also will be missing out on the stimulus package. I also on my tax return failed to add that I missed out on last years stimulus packages, is there anyway I can fix or change that? Help would be greatly appreciated.