I am single and only have $6000 income. I just filed the return to claim the tax withheld. I got a another W2 after I filed the return, it would only add $400 to my total income. Do I have to amend the return? Thank you.
Yes, you will need to amend to add that W-2. The IRS gets copies of all of your W-2s and other tax forms and will be looking for that income to be included on your return. See here for more details Do I need to amend?
So if you already filed, you'll need to wait until the IRS either accepts or rejects your return before you can add anything else.