Following their teardown of the Apple Watch last week, iFixit has posted four guides for replacing different parts of the device. There are currently guides available for replacing the battery, NFC antenna, and screen. There is also a guide for replacing the adhesive in the watch, which will need to be done after any repairs you make.
Of course, you're free to do what you want with products that you've purchased, and if repairing your gadgets on your own is something you have experience in, then by all means, follow these guides. However, for most, it's probably best to get your Apple Watch repaired by Apple or an authorized repair center.