First login to your cPanel with username, password as given to you in the mail.You can refer to How to login to cPanel. Also, if you would like to get familiar with cPanel then check Navigating around in cPanel .
After you have logged into cPanel, click on your user icon which is at top-right corner of your cPanel home page. 
You will see a drop-down list, here you have to click on the Password & Security to change your password.
Enter your old password followed by your new password. Enter your new password again. After you have correctly entered the password click on change your password now button, your password will be changed.
