You can't do much else besides pull the plug. If it causes further problems you should be able to restore as below:
Get yourself a micro USB cable (sold separately), you can restore your Apple TV from iTunes:
- Remove ALL cables from Apple TV. (if you don't you will not see Apple TV in the iTunes Source list)
- Connect the micro USB cable to the Apple TV and to your computer.
- Reconnect the power cable (only for Apple TV 3)
- Open iTunes.
- Select your Apple TV in the Devices list, and then click Restore.
On the other hand you may be able to simply start the update again.