How do you replace a headlamp on a 2001 or 2005 Outback?

Immediately behind the headlamp housing you should find a large (3 or 4 inch diameter, I'd guess) screw off cap. Once this is removed, you'll find that the bulb itself is very small and minimal. It is held in place by a clip. When the clip is undone, the bulb can be removed very easily, there is nothing else holding it in place. Putting the new bulb in is not difficult because it should have a flat side on its base which keys it to the slot. Re-apply the clip, screw on the cover, and you're done. If you have trouble (and a manual), the manual is actually quite helpful. (I was surprised by it.)