I don’t have a first gen cx-9 but If you are talking about the first attachment point (usually when in use it goes like this, ceiling to first buckle, then accross the lap to second buckle). Sometime those buckles have a slot that you need something to push into to release it. Usually you can use the male end of the seat belt to push into that slot.
Edit: based on the owner manual for the 2012 cx-9 the slot is on the Center of the front face of the buckle and you need a small object to push it ( like a key).