Get a thin walled spark plug socket. You will need a 14mm 12 point socket. I believe BMW sells one thru the parts dept. I use a
Hazet #4766-2
checkout these threads...
135i Spark Plug replacement DIY...
http://www.1addicts.com/forums/showthread.php?t=602804
n52 Spark Plug Change
http://www.e90post.com/forums/showthread.php?t=635673
DIY-Changing spark plugs on e90 330i
http://www.e90post.com/forums/showthread.php?t=174217
Does your engine idle ok? Especially on cold starts? You might look closely at your plastic valve cover... they are known to develop cracks.
Dack