It happens to the best of us. You gather all your tax documents, crunch the numbers, and double check everything on the tax return. No sooner have you dropped the return in the mail or pressed the efile button, but a new W2 or 1099 comes in the mail. Ah, now you remember. And now you need to revise your tax return. Revising your tax return is done by filing an amendment. You need to use Form 1040X, along with a new Form 1040, plus any other necessary schedules and forms.
Amended tax returns have to be mailed to the IRS on paper for manual processing. The IRS typically processes an amended return about 8 to 12 weeks, but this process can take longer during the IRS's busiest times
What Happens When You File an Amended Return
They will check to make sure your explanations are sufficient. They will check to see that you have sufficiently documented the changes in your tax return. For example, they will double check your W-2 statements, withholding, and other information reporting on your amended tax return. If the IRS needs more explanation or documentation, they will write you a letter and ask for the specific information they need. If you owe additional tax, you should mail in a check to pay the tax in full. If you are due an additional refund, expect your refund to arrive in approximately 8 to 12 weeks.