There are two answers to your question. Simple answer has been given by ObscureRobot here. I only add to make sure your source already completely summed to mono, otherwise the right channel will be dropped, as explained below.
The right channel on all five stereo jacks are connected to literally nothing (a waste if you ask me), but thankfully, since these jacks are indeed stereo we can simply bridge the left and right channels of each jack directly on the circuit board as seen in the photo. Now, the left and right channels of each input are truly summed to mono, and we have a dual-mono main out. We can use something like the Hosa YPP 111 (or if you're really daring, add a pan knob circuit —maybe I'll try that next—and use the Hosa YPP 117) or a typical insert cable (if you can find one long enough) to split the output into two 1/4 mono jacks for use with speakers. Headphones can be plugged directly in, but you shouldn't do this. Rather use a headphone amp.
Tested and works flawlessly.
I don't see why Behringer didn't do this themselves.
Reminder: Keep in mind, the output jack isn't a headphone jack, it's a line out. As mentioned in other comments, you might not get an accurate audio reference and may even damage your headphones or your hearing when plugging cans into this jack.
PS, I think the original question is pretty clear.