Changing the password of your Umbrel is easily done by going to the ‘settings’ (see the Gear icon at the bottom of the desktop view) and then selecting ‘change’ to the right of the ‘Password’ section in the middle of the screen to get a password change pop-up
Important thing to note is that the password has to have a minimum length of 12 characters, but there is no notice telling you about this.
So until you type (and repeat) a new password with at least 12 characters the submit ‘Change Password’ button will be disabled (light green and not clickable as opposed to darker green and clickable).
If you are not paying attention you could assume that you have successfully changed your password but are just not being redirected back to the settings page.