Yes, you could change the wordpress users password with phpmyadmin. The very first step is to take backup of your database before starting anything. The wordpress store the password in encrypted format and you don't need to encrypt the new password in this format.
You need to covert your password in MD5 format with the help of md5() function. You could do it through online tool like http://phpfiddle.org.
// output = 57678deb4427e2ddf83ecfbdca4bc9f4
You need to copy the output of your password ( like the above example ). Now open PHPMyAdmin and select the database used by your website. Find the wp_users table, where wp_ is the prefix used by your wordpress site. And paste the newly generated password to the user_pass field of the desired user record.