How to change password on ubuntu usb drive

I have a removable usb drive with ubuntu on it. I can't login and need to change password.I have seen many questions and answer to changing password on host drive but not on usb drive.

I tried following there commands, I can mount the drive but was lost as to how to change user password on that drive. I always ended up changing password on host drive (which is Ubuntu also).

so to sum it up, I want to be able to change the password on USB drive Ubuntu NOT the host pc.

if this has already been addressed elsewhere, please point me to it, been trying for a week now.

Thanks