How do I treat a boxer’s fracture?

Splint. See your orthopedist and he will see if it needs manipulation/surgery or just splinting for 2-3 weeks.
Typically, splinting. Boxer's fractures are usually treating with a splint or cast for several weeks. In cases with excessive angulation or rotation, the fracture needs to be realigned and may even need to be pinned. You should be seen by a physician with training and expertise in the treatment in hand injuries.