Can you amend your return after filing, before return is accepted?

Hi everyone, I thought I received all my tax-related forms and jumped the gun by filing my return several days ago. I just received another form and I read that amending your return is possible, but nowhere in my online filing do I see an option to amend the return. Do I have to wait for the return to be accepted first? I used the web-based method to file my taxes this year... not the software version. Thanks so much!