Reset the power and then start to update again. But before you do that make sure that the communication between the stick and the fixture is very strong, like under -60. I literally placed the stick just underneath the fixture's controller module by the use of a USB extension cord, I usually get -46 signal strength.
If you get the error during the update, just click on the restart button and then it will restart the update on the problem module.
In the past like about 2 years ago, this Restart button is not in the software, I had to remove the splash guard and physically pressed the reset button on the LED pad. There is an instruction file when you download the firmware, it tells you how to physically reset the pad. But I doubt that you need to do this any more.
There is one guy on another forum asking for help in updateing the firmware and he got a lot of errors as well. I pointed out the communication is the key but I believe he just ignored my post and kept asking questions for the next few days before someone again pointed out the communication issue!