  There are several contexts of non-monotonic reasoning where a priority
between rules is established whose purpose is preventing conflicts.
  One formalism that has been widely employed for non-monotonic reasoning is
the sceptical one known as Defeasible Logic. In Defeasible Logic the tool used
for conflict resolution is a preference relation between rules, that
establishes the priority among them.
  In this paper we investigate how to modify such a preference relation in a
defeasible logic theory in order to change the conclusions of the theory
itself. We argue that the approach we adopt is applicable to legal reasoning
where users, in general, cannot change facts or rules, but can propose their
preferences about the relative strength of the rules.
  We provide a comprehensive study of the possible combinatorial cases and we
identify and analyse the cases where the revision process is successful.
  After this analysis, we identify three revision/update operators and study
them against the AGM postulates for belief revision operators, to discover that
only a part of these postulates are satisfied by the three operators.
