Showing us some code would help. I'll assume you're doing this on the server.
To replace a cookie's value have you tried removing it, then adding it back with the new value?
HttpCookie cookie = HttpContext.Current.Response.Cookies[key];
if (cookie != null)
HttpContext.Current.Response.Cookies.Remove(key);
cookie = new HttpCookie(key);
cookie.Value = value;
cookie.Expires = ...
HttpContext.Current.Response.Cookies.Add(cookie);