Did you mean to post a link to another topic for more of an example?
I’m really not sure that you’re doing with your code. Since this is for a custom post type what you have in $user_id I think should just be the post_id of the post.
update_field also works better if you use the field key which looks something like field_123456789abcde. For more on field keys see the documentation for update_field http://www.advancedcustomfields.com/resources/update_field/
You should also take a look at update_sub_field http://www.advancedcustomfields.com/resources/update_sub_field/
