when I changed or updated the custom field in a product (for example the version number field), I want to send an e-mail to the user who bought that product, about the update in the product. How can I do this ?
The answer to that question hinges on being able to have a record of all of the orders made, who made them and being able to get a list of those that ordered a specific product. This would be dependent on what you are using for orders and the answer to this would come from there.
Given the ability to do the above you would create an acf/save_post action that looked for changes to the field in question and when a change is made then get the above list and use wp_mail() to send each of those people an email.