I have already filed my taxes. I received two 1099 R's in the mail today, from transferring my 401K to Vanguard. I completely forgot about them, and need to input them into 2016 tax docs.
You will need to wait to see if your return is accepted or rejected. If rejected, you may correct and resubmit. If it is accepted, you will need to file an amendment, and that will not be ready until mid-February.
When the amend instructions have been posted and form 1040X finalized, they will appear in this TurboTax FAQ.
Then, to add your 1099-R forms in amendment, please see "Where do I enter my 1099-R?"