I'm _vaguely_ considering Dremel-ing open my original Pebble to see if replacing the battery is possible. The Googleable teardowns and xrays seem to suggest there's not a lot of hope though...
You can just unscrew the back panel. I've looked for a replacement battery for my pebble but to no avail. It's actually a relatively serviceable device if you don't mind soldering. (Mines a gen1 pebble by the way)
