Warranty does not cover something being broken by user.
You of course cannot replace just the "buckle" but must instead buy a new seat belt.
For about $25 you can subscribe to WIS for a day (
http://www.startekinfo.com) and download the procedures required to remove/replace the belt. The WIS procedure will list any required special tools, although "Torx" may seem a special tool to many DIYers but isn't. It's likely a given WIS procedure will link to others for subassemblies, etc. It's sometimes a bit tough to remove plastic B pillar parts without breaking them--this is the sort of thing you pay for having someone with experience and liability insurance do the job for you.