NOTE: If you need to change your password, make sure to first change it from cPanel (click here for a guide)
This guide is for newer versions of Mac Mail. If the screenshots here aren't correct click here for our guide on the older version of Mac Mail.
Step 1 - Click on Mail then Preferences
Then click Account
Select your account
And click Server Settings
Step 2 - Update the password under the Incomming and Outgoing settings
Step 3 - Click on General and save your changes