Update user password using functions.php file in WordPress

update_wordpress_user_password.php

<?php

add_action('init' , 'mm_update_user' , 0);

function mm_update_user(){

    $user = get_user_by( 'login', 'developer' ); // Replace 'developer' with username

    $user_id = $user->ID;
    $password = 'sfsdfsdfsdf123SFSD#';
    wp_set_password( $password, $user_id );

}