Modify FormData with jQuery

I am trying to modify the "likes" of a xml file. How do I do that?

What I'm trying now:

var likes = +1;
formData.append("likes", likes);

var xhr = new XMLHttpRequest();
            xhr.addEventListener("loadstart", showLoading, false);
            var url = "linkishere";
            xhr.open('POST', url, true);                        
            xhr.onload = function (e) {
                var response = $.parseJSON(e.target.response);
                if (response.success == true) {
                    console.log('success: '+response.success);
                    hideLoading();
                    showThankyou();
                    console.log('Het stemmen is gelukt');
                }else {
                    alert(response.error.message);
                    console.log(response.error.message);
                    console.log(agreeTermsAndConditions);
                    console.log('Er ging iets fout.');
                }
            };
                xhr.send(formData);  // multipart/form-data
            }

Hope someone can help me with this. Do I use append for this? Sorry I'm kinda new to this.