There is no upgrader for a remote indexer.
If the upgrade is to a new version, uninstall and reinstall the Remote Indexer.
Usually there are no changes in a minor version. If there are changes, such as 12.5 to 12.5.2,
Take backup of all jar files from Protect\lib\jar on the Remote Indexer Machine.
Replace all of these jar files from 12.5.2 enforce server’s Protect\lib\jar folder.