change value of fields created with advanced custom fields in the front

If you want to include your own inputs on the acf_form() you should use some of its parameters to allow the form submit them. Otherwise your inputs will be out of the form. As an example you could use this code:

<?php
acf_form(
       array(
                'html_after_fields' => '<input type="text" id="id_whatsapp" name="acf[n_whatsapp]" value="acf[n_whatsapp]">',
            )
        );
?>

You can check all the parameters acf_form() accepts here: https://www.advancedcustomfields.com/resources/acf_form/

Hope this helps.