To change your password, currently you can just enter Hidden mode by tapping on the “hi” logo, mark all your hidden chats as visible, and then Reset Hidden Mode.
To Reset Hidden Mode, just go to Settings > Privacy > Reset Hidden Mode. That will start a 30 minute timer after which you’ll get the option to Reset Hidden Mode. If you have any hidden chats, they will be deleted. (So make sure you mark all hidden chats as visible before you try to change your password.)
Now, just set up Hidden Mode again by tapping on the “hi” logo, and enter a new password.