Thank You. I contacted them via email and got a reply, that the update will be available soon. However it takes already few weeks since the reply. Might be I just have to be more patient.

I successfully installed UT as second OS on Volla Phone X23 (on SD Card). The Volla Boot Manager installed the version OTA-4. In the meantime a version OTA-5 was released. I found 3 ways how to update the OS to the newest version. However none of those worked:

• the update option in Volla Boot Manager shows, that there is no update.
• the Update option in UT System Settings offers to install an update, but this just reboots the phone to Recovery mode and nothing happens.
• I found a guide to use ADB tools and command "sudo system-image-cli -v -p 0 --progress dots" but this also just rebooted into Recovery mode without further action

Can someone advice please how to update the UT running as a second OS?
Might be remounting the root filesystem into write mode and run apt update sequence? Is this a recommended way?

Thank you for any advice.