Passwords are stored as MD5 hashes. Go to
this webpage and type the user's new password in and it will spit out a MD5 hash of that new password. Then copy that hash to the windows clipboard.
In PHPMyAdmin run the following query on your database:
Code:
UPDATE users SET password='NEWPASSWORD' WHERE username='USERNAME'
Replace NEWPASSWORD with the MD5 code you copy / pasted and replace USERNAME with the user's username of whose password you want to change. :)