If you have not filed, it is best not to file until you have all of your W2's.
If you have already filed, and need to add a W2, you will need to amend once your return has been accepted.
If your return is rejected, you can just add the additional W2.
As KatrinaLO indicated above, if you need to amend, you will need to wait until Form 1040X is available after 2/12/2020 , and print and mail your return (IRS does not accept efiled amended returns.)
It can take more than 12 weeks for IRS to process the amended tax return.
Here is a detailed link with more detailed information: Amending my tax return.
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