Update existing SalesForce lead field with httpWebRequest

I have looked everywhere, but have not found a solution for this issue. I am trying to update a field in SalesForce for a lead. The way I have it sending right now is:

        string postData = string.Format("Data I am Sending");

        //send data
        var data = Encoding.UTF8.GetBytes(postData);

        try {
            WebRequest request = WebRequest.Cr开发者_高级运维eate("https://www.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8");
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";
            request.ContentLength = data.Length;

            Stream newStream = request.GetRequestStream();
            newStream.Write(data, 0, data.Length);
            newStream.Close();
        }
        catch { }

instead of it creating a new entry, I want it to update the other fields of the lead where the email address matches the data I send it. So something like:

postData = "oid=myOid&email=" + emailIWantToMatch.Text + "...";

Is this possible or will I have to use the apex api?

The Web2Lead feature can only create new leads, not update existing ones. To do updates you'll need to use either the soap or rest API

Or you can create a Force.com Site Web2Lead page (Creating a Web-to-Lead Form for Your Force.com Site) and have your controller the logic to insert/update based on email-id.