I received a CP2000 notice for underpayment of 2016 taxes, agreed with the proposed changes and paid the amount of the bill. Now I realize I should have disagreed and amended my return. What do I do now?
If you now disagree, you can amend your return and include a copy of the CP2000 notice. You can get Taxpayer assistance with this https://www.irs.gov/help/contact-your-local-irs-office.