You cannot change your password to your mail account from Thunderbird. You CAN store it, and have Thunderbird automatically use it to check your mail, but changing the password stored in Thunderbird does NOT change the password necessary to access your mail.

You will have contact your mail provider, or there may be a link on the login page, or in your provider account profile that gives you the option to change it.

Thunderbird has a 'Master Password' function that locks your local 'Password Manager' to prevent a different user, on your computer, from logging in to your accounts. It can be changed - 'Tools - Options - Security - Passwords', but it does not change the password you need to access mail.

TB-38.2 Win10-PC

I have changed my password on my webmail provider already.

How do I change the corresponding password that Thunderbird has stored for accessing the same account through IMAP? I can see the (outdated) passwords in the password manager, but I can't edit them.

You don't edit them. You remove and replace. This tells you how. https://kb.wisc.edu/page.php