If you are already logged in: You can update your password here: https://rumble.com/account/profile
If you can't login: You can request a new password here, Click on the "Forgot password" button. (You only need your registered email address.) Please note, if you're logged into your account, you will have to provide your current password in order to change it.
If you don't know your current password, then log out of your account. When you click Sign In, click on the "Forgot password" button.