One of my HID Bulbs started flickering. I tested the ballast and I had a constant flow of juice, so I decided to change the bulb. In the past, I've always changed HID bulbs in pairs to maintain the same color of light, since, overtime, HID bulbs do change light color. This is to demonstrate how to change the factory D4S bulbs. You may purchase the factory OSRAM Zenarc #66440 or go with another brand, you're choosing. I have been buying from the same seller on Ebay and have never had any issues.
The difference between the two is very minimal. Osram Zenarc's glass is tinted blue to give a nice whiteish light, but the Aftermarket 6000k are still a little bluer.
Open the hood and find the rear of the housing. Turn the cover about 1/8th of a turn counterclockwise and it pops off.
Then, turn the bulb socket about an 1/8th of a turn counterclockwise too and pull it straight out.
There are two retaining clips holding the bulb in. Push the towards the front of the car and in towards the light to release.
Install the new bulb with the big notch at the top, and reverse the order of the bulb steps to finish.
When you're finished, you'll have a nice light output again. I know this may seem like a simple change, but I know how some people are scared to try anything, and hopefully this will ease some worries about changing these yourself.
Just an added note. Many online resources say a D2S bulb will work, this is incorrect. The factory bulbs I removed were D4S. D2S need more voltage to continue running past inital start-up, and D4S use less voltage. D2S will blink out after a bit. Make sure you're replacing the bulbs with D4S
Cheers!