Changing user_nicename

user_nicename has the same value with user_login. I want to change user_nicename value only by using code snippet into functions.php or wp-config.php. Is it possible without using phpmyadmin or any plugin?

Yes, you can use wp_update_user():

wp_update_user( array(
    'ID'            => 123,
    'user_nicename' => 'value'
) );

Just replace 123 with the proper user ID, and value with the preferred user_nicename value.

The function also enables you to change the value of other fields in the WordPress users table (wp_users).