If you do a google search there’s instructions on doing a firmware update, but I’ve never tried it.
But, maybe it’s not the firmware. There’s a LOT of people using these T6 Z-wave thermostats, and not being able to set the temp would be kind of a problem. What version of Z-Wave and Home Assistant are you running. I’m curious if HA thinks your T6 doesn’t have that ability or if you have old device config file somewhere.
This don’t think this applies (because of OpenZWave), and one post is seven years old, but they had a similar issue:
All that said, I remember returing my first T6 and getting the newer one – but I just don’t have any notes why I decided to exchange them.