Yes, you need to go to US tax court in 90 days

http://www.howardlevyirslawyer.com/2013/01/12/the-three-most-urgent-irs-letters-what-they-mean-and-how-to-respond/

This letter is IRS's final conclusion. This means IRS concluded its audit, there is no other way to change their mind other than going to Tax Court.