Step 1 (Quick and Easy):
Use the lost possword feature in the Joomla front-end to send an email to your email address. This will send you a recovery link with an option to change your password.
This will allow you to easily update your password. However, if you have forgotten the email address that you used for admin user or if you didnt allow the frontend registration parameters in the Configuration Manager, you will have to use the second option mentioned below.