This guide is is for email accounts that are set up to send/recieve email through Gmail.
NOTE: If you need to change your password, make sure to first change it from cPanel (click here for a guide)
Step 1 - Log in to gmail.com and click on the Gear icon then Settings
Step 2 - Click on Accounts and Import, then click edit info for the email you want to edit
Step 3 - Update the Password then click Save Changes