From the manual. (There's also directions for doing it with the vehicle off)
Car Wash Mode (Vehicle On)
To place the vehicle in N (Neutral)
with the vehicle running:
1. Open the door while pressing
the brake, then shift to
N (Neutral).
2. The indicator should now show
N. If it does not, select
N (Neutral) again.
3. Shift into P (Park) upon
returning to the vehicle.
Car Wash Mode (Vehicle On)
To place the vehicle in N (Neutral)
with the vehicle running:
1. Open the door while pressing
the brake, then shift to
N (Neutral).
2. The indicator should now show
N. If it does not, select
N (Neutral) again.
3. Shift into P (Park) upon
returning to the vehicle.