How do I amend my tax filing after it has already been accepted?

you first have to wait until the original return is fully processed. The IRS may catch that a W-2 is missing and mail you instructions of what to do.  if not, once fully processed, when you open the app there should be a link to amend the return. 

Wait for the original return to be completely processed.

Here are steps to amend a current tax return.

TurboTax Online

1. On the Tax Home screen, scroll down to Your tax returns & documents and select the year you want to amend.
2. Select the Amend (change) return dropdown, 
3. Then Amend using TurboTax Online. 
4. If you're told to amend your return via TurboTax Desktop, follow the steps for TurboTax Desktop instead.
5. Continue through the screens,
6. Make the changes you need to make, and carefully answer the remaining questions to finish amending your return.

TurboTax Desktop

1. Open the TurboTax Desktop software for the year you're amending. 
2. Under Need to Amend a Filed Return?, 
3. Select Amend a Filed Return.
4. Select Amend for the return you need to amend. 
5. If the return you need doesn’t appear as an option, select Find a Tax File to find and load the return.
6. On the Do you need to change anything else screen, 
7. Select Update next to the info you need to change.
8. Continue through the screens, 
9. Make the changes you need to make, and carefully answer the remaining questions to finish amending your return.
10. Don’t forget to select Save As (or Duplicate on Mac) from the File menu when you save a copy of your amended return. 
11. This will preserve the tax return you originally filed. Give the file a different name from the original, like [tax year]_TurboTax Online

How do I amend my federal tax return for a prior year?