how do i reset password

It can't be done from within Thunderbird. You will need to go to your email provider's web site or web mail page to reset your password. Once you have done that you can edit Thunderbird's stored password vault to reflect the new password. In the Windows version, the passwords are stored at <Tools | Settings | Privacy & Security | Passwords | Saved Passwords. It's best to delete the entries for that server - you will be prompted to put in your new password when you next connect to it. Note that there may be multiple entries for that provider - delete all of them.