There are times when you're testing hook_update_N hooks and need to revert to a previous schema version to re-run your update hook. For example if your developing an update called my_custom_module_update_7002 and you run it there are only two ways you can re-run that update hook; 1, incrementing the update name to my_custom_module_update_7003 or 2, resetting my_custom_module's schema in the system table to be 7001.
With Drush you can quickly and easily reset the module schema using sql by using the following command:
drush sql-query "UPDATE system SET schema_version = 7001 WHERE name = 'my_custom_module';"
where 7001 is the schema you want to reset to and my_custom_module is the name of the module you're changing.