Most of the time Update(object) method is enough to update record using NHibernate. Instead you can use update query to update records, sometimes you might have special cases to write your own update query to update records on the database. Here is the simple sample how to write update query with NHibernate.

using(ISession session = SessionFactory.OpenSession())
{
using (ITransaction transaction = session.BeginTransaction())
{
var query =
session.CreateQuery(
"Update User set FirstName =:firstName,LastName=:lastName,UserName=:userName where UserId=:userId");
query.SetParameter("firstName", user.FirstName);
query.SetParameter("lastName", user.LastName);
query.SetParameter("userName", user.UserName);
query.SetParameter("userId", user.UserId);
int res= query.ExecuteUpdate();
transaction.Commit();
return res;
}
}