How to update just one column of a record with NHibernate

I want to update just one column of a record in my database. Not the whole record, but just one column (Stav). I am using NHibernate and I access through a DAO.

Rezervace_dao rezervaceDao = new Rezervace_dao();
r.Stav = 1;
rezervaceDao.Update(r);

The code above update the whole record, but that's not what I want.

Thank you a lot.