Power it off. Then hold down the wheel button while you power it back on. This will take you into a settings mode, which allows you access to a few features that cannot be found any other way. Like DAC Mode.
Because this is so much of a hassle, I don't bother. When I'm outputting to other amps, I use the XLR pre-amp setting, and volume set to 00.0dB, which is max. Then I just let the amp I'm connected to decide the real volume. Sounds perfect to me.
But if you don't plan to be doing a lot of switching between headphone out, pre-out, and DAC Mode, then you might as well just set it to DAC Mode and be done.