'Update the given xblock\'s persisted repr. Pass the user\'s unique id which the persistent store
should save with the update if it has that ability.
:param allow_not_found: whether this method should raise an exception if the given xblock
has not been persisted before.
:param force: fork the structure and don\'t update the course draftVersion if there\'s a version
conflict (only applicable to version tracking and conflict detecting persistence stores)
:raises VersionConflictError: if org, course, run, and version_guid given and the current
version head != version_guid and force is not True. (only applicable to version tracking stores)'

@abstractmethod
def update_item(self, xblock, user_id, allow_not_found=False, force=False, **kwargs):

    pass
