Matsu - one thing you must bear in mind that can often happen is that cheap, poor quality bulbs will not produce a good beam pattern. It can affect the cutoff and can end up with a heavy concentration in parts of the beam instead of an even distribution of light
If you can afford it, get the phillips, GE or OSRAM D2S bulbs as you know the quality is pukka, plus they are warranted.
With regard to changing them, its as easy as...
Unscrew the big black cap on the back of the headlight
twist the ignitor (red thing) half a turn ish and pull it off - note there is a cable that will pop out at this point that youll need to reconnect later
twist the retainer ring on the back of the bulb housing and remove
remove the bulb
fit the new bulb
refit in reverse order ;)