After you have associated an email address with your Klout account (either by signing up with an email address or linking one via your Facebook or Twitter account), you can change the password for this email by going to your profile settings and clicking Change Password.
Then, enter your current password followed by your new password and click Submit.
Note: If you enter your password incorrectly 5 times, you will be locked out of the system for an hour. You must wait a full hour before attempting to sign in again. If you are still unable to sign in, open a support ticket.