Wait for the letter. About 3-4 weeks after your refund is issued, you will get a letter in the U.S. postal mail from the IRS that tells you what they changed on your tax return, why they changed it, and what actions you need to take if you dispute the changes. So you can't do anything until you get that letter.
When you get the letter, there will be a deadline for you to take the actions the letter tell you to take. If you let that deadline pass before taking action (usually 30 days) then that's it, and you lose.
