If you haven't e-filed or mailed your 2020 income tax return, you could delete your child as a dependent. See How do I add or remove a dependent?

If you've already e-filed or mailed in your income tax return, you'll have to amend your income taxes. You won't be able to change it. It's like putting a letter in the mailbox. You can't retrieve it.

• You'll have to wait until the IRS accepts or rejects your return.
• The IRS has announced that they will begin accepting and processing 2020 individual tax returns on February 12, 2021.

If your return gets rejected, you'll be able to delete your dependent and send in the corrected return.

If your return gets accepted:

• You'll have to amend your income tax return.
•  At this time, you're not able to amend a 2020 income tax return. Check the forms availability to see the date you'll be able to amend.

Note: Generally, Form 1040-X,  Amended U.S. Individual Income Tax Return, must be filed within three years of the date the original return was filed, or within two years of the date the tax was paid, whichever is later.

The tax law does not allow a person to claim themselves if they CAN be claimed as a dependent whether they are actually claimed or not.      If you COULD claim him then he must indicate on his tax return that he CAN be claimed by another taxpayer.

You can amend (that takes 6 months to process) and remove him, and pay back any refund you received for claiming him and he should also amend and indicate that he COULD be claimed and check the second box that he was not actually claimed.   That will NOT allow him to claim himself but will allow him to claim certain educational credits that usually only the taxpayer that claims him can take the credit.  

That is why there are two boxes: 1) Can you be claimed and 2) Were you claimed.

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Online for 2018 ended on Oct 15, 2019. 2018 and earlier can only be amended with TurboTax using the CD/download version installed on a PC or Mac computer.

2019 amended Federal returns can either be e-filed or mailed. 2018 and earlier amended returns can only be mailed. It is suggested that it be mailed certified with return receipt (or other tracking service) to verify that the IRS or state receives it.

See this TurboTax FAQ for detailed amend instructions:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

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

You can check the status of the amended return here, but allow 3 weeks after mailing.

https://www.irs.gov/filing/wheres-my-amended-return