Like blue eyes says, it is not hard. You just need to remove the plastic trim pieces to expose the reel and the anchors, and you will need a big, heavy torx bit (55 or 60 maybe) with a long handle to break the anchor bolts loose. I use 1/2" drive and an air impact wrench to zip them right out. They can be rusty and/or coated with hardened undercoating. This is no job for a 6"- long 3/8"- drive ratchet handle. Take a good look at how the belts attach so that when you install the replacements, you don't get the belts twisted.
If you are getting your belts from a junkyard, check the reels to make sure they operate smoothly. Pull the belts all the way out of the reel and tie an overhand knot in the belt or install a diaper pin (big safety pin) to keep it from retracting. Lay the belts out on the floor of your local speedy wash and give them a good cleaning. Do one side at a time, then flip the belts. Keep the spray away from the reel mechanism. Hang them up outside in the sun or in the basement for a day to dry. Be careful - some washes have such high pressure that if you get too close to the webbing, it leaves a furry mark. Test this on a part of the belt close to reel that will not be seen. You will be amazed at how a filthy belt can be made to look like new. I typically clean light colored belts this way when I pick up a used car. It is a detailing item that makes a big difference.
This belt was 14 years old and really dirty- here'* how it looked after the speedy wash.