Open '''''Preferences''''', type '''logins''' inside the searchbox then click the '''''Saved Logins''''' button. This will open Tbird's Password Manager, where it saves login credentials so that you don't have to enter them all the time. Find the entries for your account, reveal the passwords and change them. You shouldn't normally have to do this though, because you should be getting a prompt to enter the correct password with a checkbox to tell Tbird to remember (save) the password.