The letter should contain instructions on what the audit is about and what you need to do. If there's not much info in the letter, you'll need to phone them.
What is the letter asking you to do?
Did you purchase Audit Defense or MAX for those years? If so, we can tell you how to contact the company that handles that (intuit.taxaudit.com).
If you did not purchase Audit Defense/MAX, you can still get free help from TurboTax Audit Support Center. At the page linked below, click the blue "Get Help" button, and follow the prompts to get a phone number.