Deleting phone update file A new system update was downloading to an asus tablet but then wi-fi was turned off half-way. Where is the half-downloaded update file? I want to delete it or prevent update process somehow before turning on wi-fi again.

I think DMcient was downloading the update.
If the update is saved to the cache partition as Bo indicates, you could reboot into Recovery mode and wipe the cache partition from there.  You can look up the key combo to get into Recovery, or if you have USB Debugging enabled and adb installed on your PC, you can connect the device and run adb reboot recovery.