Log into cPanel (your hosting administration) like this:

www.mydomainname.com/cpanel (be sure to replace mydomainname with your own domain name!)

Enter your username and password to log into your cPanel and you will see a page like this

You have access to Video Tutorials (top line with the film clipper board icon) which show you how to perform tasks in your cPanel, eg how to create a new email address.

Once logged into CPanel, look for the Mail section, open up Email Accounts.

In the bottom half of the screen that loads, choose the Account you want to change and on the Right Hand Side of the page if you run your cursor over the various icons, the 3rd icon listed is "Change Password".

Choose this option and change the password to something you would like.

Log out of CPanel.