To reset your password in Piwik, go to Piwik: below the login screen you will find a link “Lost your password?”. After clicking the link, enter your Piwik username or Email address, and enter your new password (twice). A link to confirm your password reset will be sent to your email address.
Note: for security reasons, it is not possible to recover passwords, but only change them. We highly recommend to use a strong password and to enable automatic SSL for login screen in your Piwik server.
See also the other FAQ: How do I change the Super User’s password when password recovery emails are not sent?