I already filed my return but forgot to include lottery winnings

Unfortunately, once you transmit your return you cannot change it or get it back. You would have to wait until your return gets rejected or accepted.  If rejected, you can make your changes or add information to your return. If accepted, you would need to amend your return to correct or add information to your return. 

If you need to amend your return and you're amending to claim more money, the IRS says to wait until you've received your original refund before filing an amendment. Once you've received your original refund, feel free to cash or spend it while your amendment gets processed (this can take 12–16 weeks). 3 weeks after you mailed your amendment, you can start tracking its progress at the IRS Where's My Amended Return? site.

More information regarding amending a return can be found below: 

Click the tax year you need to amend for specific instructions.

·         2015

·         2014

·         2013

·         2012

·         What does it mean to "amend" a return?

·         Do I Need to Amend my Tax Return?

·         Will amending delay my original tax refund?

·         What is the deadline for filing an amended return?

·         Can I e-file my 1040X to amend my return?

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- Answered by TurboTax FAQ to this question