Previewed at WWDC, launching in the fall.
iFixit Posts Apple Watch Repair Manuals for Battery, Screen and More
The steps required to replace the OLED display and fused glass digitizer, in simplified terms, include separating the screen from the watch body with a sharp, curved knife and opening pick, prying and removing the internal battery, disconnecting the display data and digitizer cables, and removing the screen. The steps to replace the battery are nearly identical.
Replacing the Apple Watch's adhesive involves laying the backed adhesive tape down on the NFC antenna, around the edge of the case where the screen rests, using a plastic opening tool to reconnect the display data and digitizer cable connectors, reinstalling the metal bracket, pressing the battery back into its position, and pressing the casing down firmly on the new adhesive.
While many of the tools used in the repair guides are available for purchase, iFixit has not yet stocked the necessary Apple Watch replacement parts needed to make these repairs. Given that the Apple Watch has only been available for one week, it could be some time before the website makes replacement batteries, screens, adhesive and NFC antennas available through it online store.