Thunderbird has nothing to do with your email password. You change and administer your email password with your email provider. Go to their web mail or email help page and look for a link to reset your password.

It took me a while but I finally figured out why this simple question, being asked in several ways, is not getting any direct simple answers.

We don't want Thunderbird to change our email password.

I already went thru my email provider's procedures for changing my passwords, so I already have my new password.

How do I tell Thunderbird what that new password is, so Thunderbird can access and read email with my new password?

This tells you how to remove any stored password in Thunderbird. https://kb.wisc.edu/page.php?id=19006

Done all that and when I try to download mail it DOES NOT ASK FOR A NEW PASSWORD. How to put it in manually.

The link to the procedure is how it works. There is no field to put it in manually.

when I open Thunderbird, I get a window asking me to enter my password. My password is saved in the account setup details. I have never needed to re-enter the password previously. What has happened to suddenlly require the separate entry of my email password? How can I turn off this requirement?

You cannot turn off the password requirement. It is the email server asking for the password and either you enter it or have Thunderbird save it and enter it for you.

Go to your providers web mail page and make sure your username and password works. Once you know you have a working password I would follow the steps listed in the link above to remove and replace the stored passwords to be sure they did not get changed or corrupted. If you still have issues you need to check your server settings. An incorrect server setting can cause the password error also.

My password for my POP3 email account is working OK. It works through webmail. When I log into Thunderbird at home, it is now routinely asking for a password, so I enter the password and it (slowly) opens Thunderbird for me. When I click on "Get Mail', it sits for a while before finally coming up with an error message (attached) asking for the password again. Then it will wait for some time before finally downloading new messages. The password is saved in my account and in the list of passwords inside Thunderbird. Why am I suddenly being asked to re-enter a password for everything I do in Thunderbird? Is this a Thunderbird issue or a mail provider issue?

There is an amount of confusion in the answers on this page which is not helpful. The main question seems to be 'having changed my password in my webmail site, how do I transmit this change to my accounts on Thunderbird'?

If you wish to correct a password in Thunderbird click on a bold email address in the left column and then click on 'Get Messages'. If the password is incorrect you will be given the opportunity to enter a new one. When doing so also click on 'Save in Password Manager' before you close the box.

The steps to display or remove passwords are shown clearly in graphic form in the link in 'Airmails' very helpful contribution above.

Airmail said

This tells you how to remove any stored password in Thunderbird. https://kb.wisc.edu/page.php?id=19006