Most 2001+ a-pillars have an upper restraint clip which locks the a-pillar in place. It is important to remove this clip without damaging it. Here are some tips for removing the clip:
First, peel back the rubber / fabric door weatherstripping at the top of the a-pillar. Then, find the clip. Here are two photos showing the clip installed in the pillar trim.
To release the clip you will need to slide the bottom out. Using a thin hook, grab the forked portion and slide straight out.
Once the bottom of the clip is released, the portion which ties the a-pillar to the frame of the car will release. At this point you can pull the pillar trim away from the frame. The bottom section will come off completely in your hook or remain on the pillar trim. The top portion of the clip will remain in the frame.
Here are some detailed photos of the restraint clip removed from the pillar trim:
To re-install, insert the top portion of the clip back into the square hole on in the pillar trim. Slide the bottom portion of the clip back into place, securing the clip into the pillar trim. When re-installing the pillar trim, simply pop the clip back into the appropriate hole in the frame of the vehicle. Be sure to re-install the clip exactly as was it was removed. If the clip is damaged while removing, always replace it.