Lan,
Theres no way you can resize with heat, but without damaging it. My suggest would be to cut it in half vertically, remove some of the center, then rejoin. A few of my rejoining suggestions:
1. Rejoin with a metal hinge, which would make the cut obvious.
2. Rejoin with clear resin, knowing you will have to sand a seam off.
3. Rejoin with a colored resin, making it a new decorative part of the bracelet.
Good luck!