After you file

If you have not filed yet you can change that in the MY INFO section ... Edit yourself and look for this question: 

If you have already successfully filed then you will need to amend the return to make the change.

You should *wait* until your return has been processed and you receive your refund or conformation that any tax due has been paid. (If you file an amended return while you first return is being processed it can cause extended delays for both returns if two returns are in the system at the same time). In addition, if the IRS makes any change on your original return, you might end up having to amend the amendment – a sticky process that can take a year or more).

-- Then you can start the amend process.

It is suggested that it be mailed certified with return receipt (or other tracking service) to verify that the IRS receives it. That is the only proof of mailing that the IRS will accept.

-- Amended returns can only be mailed - allow 8-12 weeks - can take up to 16 weeks (4 months) for processing.

***************************************NOTE***********************************************
Amended returns can take up to 16 weeks (4 months) to process in normal times but due the the Corona virus shutdown the IRS is not even opening mailed returns at this time and just storing them so the 16 weeks will not even start until the IRS returns to normal - whenever that will be - and there will be a huge backup of mail.
https://www.irs.gov/newsroom/irs-operations-during-covid-19-mission-critical-functions-continue
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See this TurboTax FAQ for help with amending:

https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

You can check the status of your amended return here but allow 3 weeks after filing for it to show up:
https://www.irs.gov/filing/wheres-my-amended-return