There is no conversion process.

You add the account all over again and wait for it to download all your mail from the server all over again. Once you are satisfied that the new account has all the mail the old POP one has, you can delete the POP account from Thunderbird.

The steps are exactly the same for all mail providers.

I have IMAP but I want POP3. I only yesterday downloaded Thunderbird and my emails downloaded automatically without my being given the chance of choosing POP3. Do I download Thunderbird again? Do I uninstall Thunderbird first? How do I get to choose POP3 before the emails start automatically downloading?

I have IMAP but I want POP3. I only yesterday downloaded Thunderbird and my emails downloaded automatically without my being given the chance of choosing POP3. Do I download Thunderbird again? Do I uninstall Thunderbird first? How do I get to choose POP3 before the emails start automatically downloading?

First Depends on whether your mail provider supports pop. If they do, remove the imap account in Thunderbird, then restart, then start the process to add the account back. At the second or third screen you can override the imap default and choose pop. https://support.mozilla.org/en-US/kb/automatic-account-configuration