If you have already filed your tax return you need to first wait and see if your return is accepted or rejected. If rejected, you can go back and add the Form 1095-A, then re-submit your return.
If accepted, you need to wait until you receive your tax refund and then Amend your tax return using the Amendment Form 1040X.
Here is an FAQ with additional instructions on Amending your 2016 tax return.
https://ttlc.intuit.com/replies/5114064
