O2 has released a Windows Mobile 6.1 upgrade for the Xda Stellar. The new ROM is available for free but you will need to provide your device’s serial number to download it.
Given that the Xda Stellar is based on the HTC Kaiser design, hopefully a similar upgrade is headed to other Kaiser devices soon.
Note: The update will replace the current ROM image and will erase partnership and all user data and programs on your device. Be sure to back everything up before you start.
Source: PocketPC Thoughts