Changing email password in Mojave

Passwords for mails are always changed on web server account , the apple mail application is a configuration with web mail server account , there is no settings to change passwords from IMAP accounts .

It depends which mail services do you use , for gmail accounts see this link https://support.google.com/mail/answer/41078?co=GENIE.Platform%3DDesktop&hl=en

You have to go to your email account host's website. If you're using Gmail, go to https://account.google.com