How do I delete a W-2 that I put on my taxes after I already filed

You'll need to wait for a bit.  Once you have transmitted your return you must wait for the IRS to respond with "Accepted" or "Rejected".  If your return is "Rejected" for some reason you will then have the opportunity to make any changes you need/want to your return before retransmitting.  Those changes could include "Deleting" or "Editing" form W2 entries in the "Wages & Income" section.

However, once your tax returns are "Accepted", you cannot make changes to the filed returns.  Instead you will need to wait for your return to be processed and then complete an amendment to delete or change any information.  

Later in the filing year, usually late February to early Mar, the "Amend" option is added to TurboTax. When you sign in to your account you will see the "Amend" option.  You will click there once available and follow  the prompts to complete the amendment process.  

Finally, the IRS attempts to reconcile your tax return with forms W2 issued to your social security number.  If there is a discrepancy many times they will catch those and send you a letter with instructions.