You can check when the update will be available in your region here
You should receive a notification on your device when the update is available and you can obtain the update by connecting your Lumia 800 to your PC using the Zune Software or to your Mac using the Windows Phone connector.
The update change-log includes
- Improved battery life (25-30 hours with heavy use),
- Improved base sounds
- Improved call quality
- Soft key illumination at all brightness levels
It doesn't seam like the new scene modes for the camera have been released with this update. Nokia has also stated that the Wi-Fi tethering feature will be available through a later update.
If you want details on how to update your Nokia Lumia 800 with the latest firmware you can check out the following instructional video