How do I amend my tax forms? I filed in March forms were accepted mid March and I got my 2nd stimulus check the last week of March. How do I amend them 2 account for $600

The get my payment thing still just says accepted. How do I or do I even need to amend my forms. And I live in Illinois claimed single and still haven't received my refund or the rebate tax credit. What do I do to expedite all this?