Drupal - Resetting/reverting a custom module's update 'N value'

The schema_version column in the system table contains the version of the last successful update. Manipulate that value and you can re-test your update.

The name column is the machine name of the module, same as the info file is named.

This query will do the trick for you.

UPDATE system SET schema_version = XXXX WHERE name = 'name_of_module';