To reset your password in Matomo (Piwik), go to Matomo: below the login screen you will find a link “Lost your password?”. After clicking the link, enter your Matomo 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 Matomo server.
See also the other FAQ: How do I change the Super User’s password when password recovery emails are not sent?