Looks like you twist the front, pop the lens out, change the bulb, and then put it back together in reverse.

Here's the paperwork: http://www.petzl.com/files/all/technical-notice/headlamps/E69P%20DUOLED5%20E69500-L%20.pdf

I'm sure it'll make more sense to you than me, since you're familiar with the light.