Once Tom Nook's store's name changes to Nookington's, go through the sliding doors and walk straight ahead. Go left at the stairs; you should see a barber pole. Walk into the barbershop, pay for a haircut, sit on the chair with the hairdryer, then answer a few of Harriett's questions. After getting your hair dry, you should have a new 'do! WARNING: you may end up with really funky hair!