First, you will need to wait to see if your return is accepted or rejected. If it is rejected, all you need to do is to log back into your return, add the missing W-2 and fix the errors that caused the rejection and then re-file the return.
If your return was accepted, you will need to file an amended return using form 1040x. Click the following link with instructions and when you can file the 1040x: https://ttlc.intuit.com/replies/3288565
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