Changing your nominated password directly through phpMyAdmin will not work since it is automatically encrypted by WordPress MD5 hash function, follow this instruction instead.
I presume you already know how to go to your phpMyAdmin, just contact your Hosting Provider if not.
Go to wp_users (database table) and click edit icon like what you can see in the screenshot below.
The next screen will show you a user fields as shown below but don’t put your nominated password directly since you need to encrypt it using MD5 Hash Generator tool found at http://www.miraclesalad.com/webtools/md5.php and you can get similar character format like what you can see below:
MD5 Encrypted Characters
Then paste it to password field and click Go.
MD5 Hash Generator Tool Screenshot