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?

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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

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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.