I accidentally input the 401k amount on my W-2 in the deduction session. How can make change?

If haven't filed your return and you entered the 401k amount from your W-2, box 12 (code D) accidentally as an IRA contribution in the Deduction and Credit section then you can delete the entry with these steps:

1. Login to your TurboTax Account 
2. Click on "Search" on the top right and type “IRA contributions” 
3. Click on “Jump to IRA contributions"
4. Deselect the IRA and click "continue"
5. Confirm that you want to delete the entry.

Another way:

1. In the left menu, select "Tax Tools" and then "Tools".
2. In the pop-up window Tool Center, select "Delete a form".
3. Select "Delete" next to the IRA Contributions Worksheet in the list and follow the instructions.