If you've forgotten your email password, the first step is to utilize your email provider's password recovery options. Typically, this involves going to gmail.com, yahoo.com, OR whatever your provider is, then clicking "Forgot Password" on the login page. After that, you are just following the prompts to verify your identity. Once that is done, you can re-enter it into thunderbird. You Can go to Settings -> Privacy and Security, Saved Passwords to remove the old one and add the new one.