With the release of OpenELEC 4.0, XBMC 13 (Gotham) is now available! For those of us driving a media center with a Raspberry Pi, this is great news (there’s significant performance improvements on the RPI).
Anyhow, it turns out that the upgrade process from OpenELEC 3.x is extremely simple. Just extract the downloaded package, and in the target folder you’ll find four files - KERNEL, KERNEL.md5, SYSTEM, and SYSTEM.md5 . Using Windows Explorer, browse to your media center (eg, \mediacenter), and drop those files into the upgrade folder. Reboot the media center, and it automatically upgrades.
On mine, there were a few PVR addons that enabled themselves I didn’t want, so I just disabled them and rebooted again, and all appears fine.