You should amend, as soon as it becomes available. Yes, you can wait for an IRS letter, but if you owe, there may be penalties that accrue until you get the letter.

Just attach the single W-2.

Yeah my refund will go down a little and I guess I will have to return the difference to IRS. Thank you!

@ColeenD3 One more question. I started amendment process and once I added the missing income from W-2 form my state refund went down to 0 (before it was 40$). Should I still amend state return or maybe I can just send them a payment of 40$?

When you amend your return, you will just be adding the omitted W-2 to the original return you filed so it will be a "complete" return for all your income and deductions.

Generally, we do not suggest it file an amended return until your initial return has been fully processed.

You can complete the amended return in TurboTax.

The states and IRS share information, so if you amend your federal return and it affects your state return, you need to also amend your state return.