Call Surefire. They probably send you a new head (if you're in the US, at least).
I do not have a Z2; if the head is similar to the C2 and G2, then yes, the window can be removed fairly easily, as it just hold by a retaining ring accessible from the front. You can use needle-node pliers to turn the ring, but be careful, as it is easy to slip; not a problem with the broken window, but it's easy to scratch the new one.