What error message do you receive? Normally, Thunderbird will prompt for password. If you need to remove current password, you can - go to tools>settings>privacy&security - click saved passwords and then show passwords - highlight each row (incoming and smtp) and there is a dropdown menu to change password.

If that doesn't work, then highlight both rows and delete them. That clears everything so that the next attempt to retrieve messages should force the prompt for password.