in order to create and update new custom fields, you need the following code :

function custom_field_get_post_meta_cb($object, $field_name, $request){
// return get_post_custom();

$metas = get_post_custom($post_id);

foreach($metas as $key => $value) {
if(sizeof($value) == 1) {
$metas[$key] = $value[0];
}
}
return $metas;
}

function custom_field_update_post_meta_cb($value, $object, $field_name) {
$postId = $object->ID;

//return update_post_meta($postId, $field_name , $value);

foreach ($value as $key => $Data) {
update_post_meta($postId, $key , $Data);
}
}
add_action(‘rest_api_init’, function(){
register_rest_field(‘post’, ‘custom_fields’, array(
‘get_callback’ => ‘custom_field_get_post_meta_cb’,
‘update_callback’ => ‘custom_field_update_post_meta_cb’,
‘schema’ => null,
)
);

});