I have my email with .Mac and need to change password

Your .Mac account uses the same password as your .iCloud account and by extension your Apple ID.

See here to change the password for your Apple ID: link-> Change your Apple ID password - Apple Support

Mail has two major working parts.

There is the email reader on your Mac.

and the Email SERVER that sits on the internet, sending and receiving emails on your behalf.

To change the password for your mail account, you need to discuss what the password should be changed to with the email SERVER.

https://www.lifewire.com/how-to-change-your-icloud-mail-password-1172268

NB>> I also have a .Mac email account.

However, when I have to fiddle with it, I am QUICKLY reminded that the underlying account is .iCloud, and the .Mac suffix is only maintained as an illusion "for my convenience" under certain circumstances, and not all the way through everything.