Well, that describes a dialogue that you see when setting up an account from scratch. It isn't relevant to making adjustments to an account.

Passwords are stored under Tools|Options|Security|Passwords→Saved Passwords.

If you need to change an existing account's settings (e.g. name, server, port, security options, authentication etc), you'll need to go to Tools|Account Settings.

ral said

A thunderbird support page says there is a 'password' box under 'email address in tools/account settings. Not true. Under email address is 'reply to address. I can't find a password box anywhere in account settings. The page is: https://support.mozilla.org/en-US/kb/manual-account-configuration. Don't tell me I have to uninstall and reinstall thunderbird to get to it.

Thank you, Zenos!

(Actually, I found the answer myself at Mozillazine http://kb.mozillazine.org/Setting_and_changing_email_passwords - by googling "thunderbird email password".)