Funny - I just answered this question on another forum. Mine are the 306BB servos so I guess yours may well be as well. They "split" around the body, just under the fixing lugs. Firstly, remove the two printed labels on each side of the servo. They just peel off.
Get a knife and push it into the casing just under a lug and gently twist. Do the same under the other lug. Kepp repeating this until the main servo arm bearing clears the top casing and presto! Be careful because as soon as the bearing is clear the top can jump off along with all the gears and spindles!
Hope this helps!