Go to your local service center or dealership and have them look at your seat belt. Fixing a seat belt should be done by auto care professionals. Safety is important and the correct fix needs to be done to your vehicle.
Don't! Seat belts are important safety devices and need to be in good working order to function properly. If the seat belt has been torn or cut, take the car to someone qualified to repair it. Do not attempt it yourself.
IT would be much safer to replace instead of repair a damaged belt. To locate a suitable replacement call your local car dealer, junkyard or parts store. The simply unbolt the old one and install the new one.
This is not a job to do yourself. Someone's life depends on that belt functioning correctly in a crash. Take it to a dealer of that vehicle to make sure you are getting exactly the right thing installed. I had to do this after a dog chewed my belts.