Remove Airbox / Battery depending on side (both recommended for colour matching)
3 torx screws to access the bulbs.
Not sure why you would need to do both, especially at the cost of these bulbs 
But then again ive never had to replace any of my hids, so what do i know
Bulbs, dear at retail, but i've just had one blow, got a pair for £42 from ebay.
As for changing both...
These lamps have a 3000h life, their brightness decreases over time.
Elite Pete lent me an old bulb of his, as mine failed just before MOT, and I both wasnt sure it was deffo the bulb, and didnt have time to buy new bulbs from a cheaper source (vauxhall want £70+ per bulb)
I Install Pete's other old bulb, completely different light output to my other one, made the headlight effect very lopsided, I couldnt have lived with this!
My new set arrived, I had ordered 6000k colour bulbs (original are 4300K, a slightly yellower colour), put the 1st one in (replaceing pete's) so I now have 1 very new, 1 very old and the difference was probably enough to fail an mot, yes similar light output is a requirement!
Replaced the other as well, and holy sh1te, what a difference, so much brighter!
Hope this helps.