The process of updating the firmware of a CardioMood is very reliable, but it still might happen that it fails. This is mostly due to the breaking of the Bluetooth connection between the CardioMood and the phone, for instance when you move away from your phone or disable the Bluetooth before the end of the update process.
Don’t worry though, the CardioMood can recover from this. Refer to the troubleshooting chapter of this article to know how.