In most cases you should click on the "Change your password" link under the login form. You will be asked to enter your old password and confirm your new password. The next time you log in you can use your new password. Be sure to make it something you will easily remember and something which is secure eg. includes numbers as well as letters.
If you login to PebblePad using your institution username and password then you are unable to change just your PebblePad password.