Mediawiki/Change User Password
How do I change a user's password?
Assuming that $wgPasswordSalt is set to true (the default), you can use the following SQL query:
UPDATE user SET user_password = MD5(CONCAT(user_id, '-', MD5('somepass'))) WHERE user_name = 'whatever';
Where obviously "somepass" is changed the password you want to set and "whatever" is changed to the user name as it is listed in the table "user".