@futaba How did you file your 2020 return? IIRC, the IRS isn't accepting e-filed returns until mid-February, so if you used something like TurboTax / FreeTaxUSA t may be possible to "pull back" your return, make the corrections, then re-submit.
If that's not an option, then sure you could submit an amended return (1040X) including the info from that W-2, but you do have to wait until your original is fully processed. And I think I've heard that you can e-file amendments starting this year, previously you had to do it by mailing paper copies.
Practically speaking, there's a pretty small chance it'll matter. Your tax bill is generally not directly calculated, it comes from
a big lookup table based on your filing status and the "taxable income" number from your 1040. Most of those are in $25 or $50 chunks, so if this extra $9 doesn't move which block you're in, it has zero effect on your tax bill.
There's also a decent chance that once all your tax forms (including this W-2 you missed) makes it to the IRS, they'll catch that difference, automatically account for it (both owed tax and any withholding from the W-2), and just send you an adjusted refund. Small errors like this are not really a big deal. If your return was missing tens of thousands of dollars of unclaimed income, that'd be more of a potential problem.
