convert short term user acess token to long term user acess token using c sharp

I got short term user acess token in the code below.

Kindly guide how should I convert and get long term user acess token.

var fb = new FacebookClient();
dynamic result = fb.Get("oauth/access_token", new
{
    client_id = "done",
    client_secret = "done",
    grant_type = "client_credentials"
});

fb.AccessToken = result.access_token;
Label1.Text = fb.AccessToken;