$meta_query_args = array(
    array(
        'key'     => 'access_code',
        'value'   => 'abc',
        'compare' => '='
    )
);

$args =  array(
  'meta_query'   => $meta_query_args
);

$users = get_users( $args );

// User Loop
foreach ( $users as $user ) {
  update_user_meta( $user->ID, 'association', 'XYZ Corporation');
}