This will revert you to the last, previous firmware. (You can actually re-update to the latest firmware at the update nag screen which will then appear whenever you start up Tablo, if you want, and go back and forth multiple times if you like.)
Don’t do this if you are not comfortable working with equipment like Tablo, routers, etc. To roll back to the previous firmware (of course both I and Tablo are NOT responsible if you try this without being advised to do so by them and have an error): press and hold the Tablo’s blue button and IMMEDIATELY unplug the power to the device while still holding the button down; then replug/power the Tablo, still holding the blue button and in a few moments as the flashing blue Tablo blue light turns solid IMMEDIATELY release the blue button.
IMPORTANT: Note that Tablo in the previous bank rollover posting urges not to do this without need and care and talking with them, since if you press the blue button too long before unplugging or after replugging there is a chance you will factory reset the device and lose all your recordings and settings, as you would if you held the blue button down for 5 or more seconds to do a deliberate factory reset. If, on the other hand, you accidentally release the button before you unplug the device you may just reboot the Tablo without changing anything (like a deliberate short press to cause rebooting), or if you release the button a moment too soon (while the blue light is still flashing after the Tablo is repowered) you may not change anything. In the latter two cases you can just try the procedure again.
I don’t know what the effect would be if you did this while recording or otherwise accessing the Tablo, but you would certainly lose the recording and/or signal and maybe damage your recording database; I made sure I wasn’t otherwise using the Tablo when I did this.