AJAX function to update a selected ACF-field

Hi, I'm using Wordpress with ACF. I've made a contact form with CF7 and I want the user to be able to pick first a category and the a Product from two different lists. The specific custom post will have a field that needs to be uppdated when the user sends an email, which actually is an reservation. In short terms, i want to set the selected item as "sold" when the user clicks on "submit". I belive this can be made using AJAX and a function attatched to "on_sent_ok" in CF7. So the selected custom post, with a field which will be changed from yes to no. Feel free to pm if you have any questions. Cheers, Markus