Normally, Drupal does a great job of resetting the password for an account as long as you know either the username or email address associated with the user account.

However, if you have lost both, or perhaps the Drupal installation has a non-functioning email interface, do the following (requires accessing the database using phpMyAdmin):

In phpMyAdmin, browse the users table.  You will see the user names and associated email addresses.  if the email interface for the site is working, simply enter the appropriate email address or username in the "Request New Password" page (http://example.com/user/password), and Drupal will send an email that has a temporary login link so that the password may be reset.  If the desired account has an old (non-functional) email address, you can update the email address using phpMyAdmin and have the temporary login link sent to the new email address.

If for some reasaon Drupal can not send an email to reset the password, browse to the desired user account and click edit icon, and enter a new password in plain text in the "pass" field, and in the Function column choose MD5, then click Go.