1) Login to phpMyAdmin and select your WordPress databse.
2) On the left side click wp_users.
3) Click the edit link in the users row for the user you want to change the password for. As a not you can reference the user_login column for the username.
4) Go to the user_pass row and first slect MD5 in the function column then type what you want the new password to be in the value column.
5) Scroll down and click the go button.
Now you should be albe to login with the new password you set for that username.