Lost password of qube

I have standalone Ubuntu qube with no network access, from which I forgot root\administrative\sudo password.

Normally, I would boot from other OS and chroot. Or boot in recovery mode as boot option.
But I can’t figure how to achieve any of these for qube.
I tried to start it in debug mode with HVM (to get boot options) - few lines of text and blackscreen.
I tried to boot from CD (of ubuntu installer) - system just boots as normal.

The question is: how to change lost root password on debian-like qube?

Mount the LVM partition and edit the files. Type lsblk to verify the exact name (use grep to help you locate it).

This article explains everything: How to mount LVM images | Qubes OS