Hi @mswas
There are few issues with your code:
1. As mentioned by @ACF Support, you should use $field_name in the get_field function.
2. Make sure you are loading from the correct post by adding in the $post_id parameter to the get_field function. This parameter is given to you in the acf/update_value filter.
3. Your update_field function is not targeting the user. You need to find the current user (with some WP code) and use that ID to target the save function.
Thanks
E
