To modify the metadata of a document set, use a SharePoint List Item Event.
We want to update a SharePoint List Item (yes, a list item, not a document, even though the document set is stored in a document library).
When configuring the Update List Item page, do not use the “Browse” button to specify the List Name, it will show just lists and no libraries. Use the ellipsis and select the library (that’s the trick). The List Item ID could be used from the SPIntegrationData object.
Now configure, what should be changed in the metadata. That’s it.