When you receive an IRS notice you usually have a set amount of time to respond if you do not agree with the changes. You can call the number on the form to get more information, and yes usually they will tell you to amend. If the back taxes were due to someone else filling a fraudulent return, you should contact the fraud department with the IRS and fill out the necessary paperwork. If you filed a return and the IRS came back with income you left off which you determine to be fraudulent then same thing should be done. Here is the link to help you get this worked out with the IRS.
https://www.irs.gov/individuals/how-do-you-report-suspected-tax-fraud-activity
