Change field value in lightning component

I'm trying to change the value of the field Instagram_followers__c of my Influencer__c object in my lighthning component. It's a really simple component: a refresh button wich check the number of followers when we click on it. But I don't know how to change the value of the Instagram_followers__c field.

My componenet :

<aura:component implements="flexipage:availableForRecordHome,force:hasRecordId" access="global">
<aura:attribute name="recordId" type="Id" />
<aura:attribute name="Influencer" type="Influencer__c" />
<aura:attribute name="message" type="String"/>
<force:recordData aura:id="Influencer__c"
              recordId="{!v.recordId}"
              targetFields="{!v.Influencer}"
              layoutType="FULL"
              />
<p>Followers count for <b>{!v.Influencer.Instagram_account__c}</b> : <lightning:badge label="{!v.Influencer.Instagram_followers__c}"/></p>
<p>{!v.message}</p>
<lightning:button label="Refresh" iconName="action:refresh" title="Refresh followers count" onclick="{! c.refreshInsta }"/>

My controller:

({ 
refreshInsta : function(component, event, helper) {

    component.set("v.message", "Loading...");

    var instaUserName = component.get("v.Influencer.Instagram_account__c");

    var url = "https://mysite.com/GetFollowersCount.php?user=" + instaUserName;

    var request = new XMLHttpRequest();
    request.open('GET', url, true);

    request.onload = function () {
        var followers = this.response
        if(followers == 0){
            component.set("v.message", "An error occured. Please try again later (in 5min)");
        }else{
            component.set("v.message", "New followers count: " + followers);
            component.set("v.Influencer.Instagram_account__c", parseInt(followers))
        }

    }
    request.send();


}
})

But it doesn't work, I see the button on the page, I click, I wait, the followers count appear on the page (on the v.message field) but the record isn't changed.