By username, I presume you mean the email address. That is on the account pane and in the Outgoing Server (SMTP) pane. To change the password, I suggest going to settings>privacy&security, click 'saved passwords' and then 'show passwords', followed by highlighting the two rows for the account, rightlclick and delete. Then, when you request new messages, you will be prompted for the password, which TB will then store for you.