You would need to use a meta query for this.
$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');
}