How to change Email password

What do you mean by "the info file'? When you write "it won't work on my iMac", do you mean in the Mail application? If so, open Mail's preferences, select the "Accounts" panel, then look for the password for the "incoming mail server". There should also be a password for the "outgoing mail server". To see that, pull down the list named that way at the bottom of the window, select "Edit SMTP Server List", select your SMTP server, then select the "Advanced" tab. If there's a password there, then change it also.

Your process does not work for gmail accounts. I opened the apple-mail preferences and selected accounts but the incoming Mail Server does not have a password field like the one in my icloud mail account dialog box. The only way I was able to change the password for the gmail account was by deleting the google account in the mail preference dialog entry. Then, I recreated the google gmail account in the apple-mail preference dialog box. I know it sounds crazy but it was the only way I could change the password in the apple-mail for my gmail account. It all worked correctly after.

I simply sent myself a "Test" email and entered the new password when a dialog box opened asking for it. Problem solved.