See this document https://images-eu.ssl-images-amazon.com/images/I/81suGzfRC7S.pdf
(because otherwise you won’t believe me).
# Plug and unplug power cable 25-30 times, until the power light flashes.
# Leave to charge for a few hours
# Meanwhile, put Ultimate Ears app on your phone.
# Unplug from power, switch on with button on top of Boom unit. Quickly connect with Bluetooth to your phone.
# Open UE app, which will prompt an urgent firmware upgrade. Download and install the upgrade (does this automatically).
# Never let it get to 0% battery charge again.