Oh ok, thank you. I got it working.
So in order to change the user password and update it, you have to choose the “PERSISTENT mode SYSMAINT session” and then you can use sudo in the Terminal. And I still just preferred to use the “sudo passwd user” and “sudo apt update && sudo apt upgrade -y” commands in Terminal to change the password and update Whonix. It’s what I already know how to do so I stuck with it.
Can one of you please update this page with up to date information Post-installation Security Advice