What's the make and model of your AVR? Of your speakers?
-- Edit --
Never mind, I see from another thread you started that you have a Pioneer VSX-1019. The instructions on pg. 36 of the owner's manual seem pretty straightforward:
Switching the speaker impedance
. . .
1 Switch the receiver into standby.
2 While holding down ENTER on the front panel, press STANDBY/ON.
The display shows RESET ◄ NO ►.
Use TUNE +/– (or ↑ / ↓ on the remote control) to select SPEAKER ◄ 8Ω ►, and then use PRESET +/– (or ← / → on the remote control) to select SPEAKER 8Ω or SPEAKER 6Ω.
• SPEAKER 8Ω – Use this setting if your speakers are rated at 8 Ω or more.
• SPEAKER 6Ω – Use this setting if your speakers are rated at 6 Ω.