i just done this a few weekends ago, as my original trim had cracked.
if you unclip the plastic cover from the seatbelt height adjuster, that reveals a large torx screw which when undone frees the belt from its top mount.
if removing the trim completely, you'll also need to take the outer seat trim off of the side of both front seats to reveal the torx screw that secures the belt to the seats. With this off you can then feed the belt and the buckle, plus the plastic trim that sits half-way up the b-pillar, down behind the pillar. lastly undo the two screws at the bottom of the pillar, and then starting at the top, pry the trim off.
EDIT: forgot to add, you also need to remove the door seals, not completley - just where they attach to the b-pillar.