How can I go back in my tax return and fill out the 1040 form? I need to receive my first and second stimulus checks, but I already filed and I’m not sure what to do.

I had just turned 18 last year and didn’t qualify for the stimulus then, but I qualify now for all three. I need to file this form in order to receive the money, but I already filed my taxes. Is there a way for me to fill out this form still and receive that money? Please help!