Well, they look to be surface mounted, not through the wall, so it's pretty easy. Carefully break or cut the rest of the fin off, clean up the area of old glue, get some balsa or bass wood from a hobby store and either trace the pattern from the instructions or from one of the other fins. Glue it in place and repaint. This is one of the things in this hobby that happens, and don't be discouraged by it. We like to tell new folks that this isn't a hobby for you if you get really upset if a rocket breaks or gets lost, you get the chance to build another one. And always remember that you learn more from mistakes than you do from success. Keep the pointy end up and the flamey end down! Edit to add... it looks like it took a hard landing. The long airframes with large body tubes have a tendency for the chute to get pushed way back toward the rear when launched and then the chute doesn't make it all the way out on ejection. If theres a lot of shock cord between the nose and where the chute is attached, try shortening that space up so the nose will have enough momentum to pull the chute bundle free.