Migrate : Update not working
If you are using the Migrate UI, at the bottom of the page there is an Options slideout with the Update checkbox. If you are kicking off your migration programmatically then call prepareUpdate() before calling processImport().

migrate_static_registration(array($machine_name));
$migration = Migration::getInstance($machine_name);
$migration-&gt;prepareUpdate();
$result = $migration-&gt;processImport();