As with all of NAD's amplifiers and receivers, your T-765 doesn't have a switch for setting speaker impedance. It's automatic! Called "PowerDrive", it incorporates two power supply rails. One runs continuously, while the other kicks in only when required. Speaker impedance varies with frequency, so the impedance as marked on the back of your speaker is a nominal figure only.
I believe that other, more knowledgable, members of this forum would advise anyone owning an AVR with an impedance switch to leave it at 8 ohms anyway.