You can change your PIN or Security Question / Answer on Sprint.com. First, make sure that your are already noted as the account holder on your sprint.com login. Here is a blog that will show you how to check that.
After logging in, click on My Preferences and Under All about about my account, click on the link which says My account PIN. There is a way to recover you PIN if you don't remember it, or simply change you PIN or Security Question / Answer on this page.
Each time your PIN or Security / Question is changed, and email will be sent to your designated email address at the bottom of this page. You can change the email address for that notification on this page as well.
Below are screenshots of the steps for your reference.