Kia rear seat belts

What vehicle and what year?

Varies greatly for each!

how to release the back seat? So I can try and fix it?

if it a 98-2001 sephia:

Pull down the back seats, and use a ratchet with a 10mm head and unbolt and remove the fold down seat sections. They are on a shared bracket, pull up on loose side and pull out left or right depending on which one you are removing.

next remove the lower seat by pushing on the 2 release buttons on the bottom leading egde, and pull up. The 2 buttons are hidden for anestedics reasons, so to find them run your hand across the edge and you'll feel them. And they are sometimes hard to push in, just warning ya. When you pull the seat up, pull all the seat belts out of the respective slots in the seat and remove the seat.

You now have acces to the 2 side bolsters. At the bottom of both side bolsters is a 14mm bolt head which holds the bottom of the bolster in and the bottom of some sephia's seatbelts. Knowing that, it will be difficult for most average people to remove. Its damn tight.

Once you remove those 2 bolts the bolsters will slide up, giving you access to the seat belt mechanism and rear deck.

I know it looks like a lot of work but it only takes 10 to 15 mins to do.