How do i upload personal documents like birth certificate/SS card.

You do not enter your birth certificate or Social Security card on your tax return.  Why do you think you need to do that?  Did you receive letter from the IRS about verifying your identity?

last year i had to send my 2 children birth certificate for Earned Income Tax Credit,

and refund was delayed, so i figured i would submit in case they needed it again

Also my federal return was rejected because "form 8862 must be present in the return"

Please see the FAQ below on how to enter Form 8862 after your return was rejected:

https://ttlc.intuit.com/community/rejections/help/e-file-reject-ind-046-01-form-8862-is-missing-from-the-tax-return-and-it-is-required-to-be-able-to/00/26189