You need to use the ACF function update_field() and you need to use the field_key to update.
https://www.advancedcustomfields.com/resources/acf-update_field/
When using a relationship field or a post object field then the value of the field will be post ID or an array of post ID representing the company post that is related. For a relationship field this will always be an array. For a post object it will be a single value unless you have enabled multiple selections and then it will be an array.
