I believe passwd is a UNIX command across all Linux systems, including Qubes OS which is based on-top of Fedora/Xen. So this is "perhaps" what you're looking for.
I looked around, and I found this different Qubes password issue fix which includes the use of passwd https://groups.google.com/forum/#!topic/qubes-users/ckDuXUz1qOw it seems to indicate that passwd is used for Qubes too, not surprisingly, but I can't tell with certainty so I had to look for confirmation.
Try "man passwd" or "passwd --help" in dom0, it should give you instructions. I believe there is no difference here between Fedora and Qubes running on-top of Fedora, when it comes to passwords, however, be careful as I do not know this for certain.
You might want to backup everything first if you're doing something that can potentially lock you out of your system. If you are in a hurry, then only backup the AppVM's or the most important AppVM's, just be sure to keep your important data safe. Too many take the risk and some day end up paying the price.