This is covered in the FAQ -
Q - How do I access the onboard menu on the Champion sabers?&
Q - I want a saber with sound, but I do not always want to hear the sound... is there a way to toggle the sound off?
But here's the basics:
hold down the power button for 3 seconds while the saber is powered off. You will hear it enter the menu, and give you options. Press the button once to cycle through the options. Once you have a setting you like, hold the button down for 3 seconds to advance to the next setting. Once you have reached the end of the menu, it will tell you that your settings have been saved, and the saber will turn on.
setting the volume control to "Mute." You will notice there is still a very slight, barely audible hum coming from the saber when it's muted, this is normal.