I'm rusty on MySQL so it took me a while to go through this. I'm leaving this here for future internet travelers. It's based on Brad's solution, so read through that first, then you can use these commands. If you forget what your database user/name is, go into Craft > App > Config > db.php
Login to MySQL on the command line:
$ mysql -u craftuser -p
Choose the corrrect database:
mysql> USE craftdatabasename;
Make sure that the tablename is craft_users (it might be just users if you didn't use a prefix):
mysql> SHOW TABLES;
Show what the current hash is (you'll want to do this from your local install as well to compare and be able to copy in the new hash)
mysql> SELECT username,password FROM craft_users;
Update it to the new hash:
mysql> UPDATE craft_users SET password='passwordhash' WHERE username='admin';