Basically, follow the instructions here,
especially the part about the backups and where to download the flasher (the 770 flasher for OS X works with the 800 too) and where to download the image to flash.
However, there is one crucial step not mentioned on that page. After you do your backup and download the flasher and the image, you need to unplug your tablet from the wall and from the computer and turn it off. Take out the SD card. Then, run the flasher. When the flasher says it’s waiting, plug in the device to the USB. Hold down the home key (the one on the front with the poorly drawn house on it) while pressing the power button. Make a note of holding down the home key, because it’s rarely mentioned in documentation.
For some reason, I had to try running the flasher app more than once. The first time, it had a USB error. The second time, I tried to run it with the tablet already in the ready-to-flash state. I would have turned the tablet back off to try all this, but it doesn’t seem to want to turn off before flashing when it’s in that state. I don’t know what happens if you have it like that and can’t get the flasher to work. Does it return to normal if you pop out the battery?
After the flash is complete, turn the device off, pop the SD card back in. With mine, it asked me for some date and time and the restore application popped right open. I restored everything, regardless of date. The utility does not backup software and everything on the device is wiped, so I’ll have to re-download everything. Alas.
Reviews of how well it works post-upgrade will be forthcoming.