Call/write Princess customer service and tell them. As long as your departure mid cruise doesn't contravene the PVSA (e.g., embarking at a U.S. port and disembarking at another U.S. port), and you're willing to pony up the gratuities due among other things, it should be no problem.

Saw quite a few people getting off a Princess cruise at Victoria, BC (coming up from California) rather than continue to Vancouver only to be booted off the next morning.