How do I merge keywords in Apple Photos?
From this Apple Discussion:


  To merge two keywords "key1" and key2" I would create a smart album with the rule "keyword is key1" or "keyword is key2". Then assign the new merged keyword to the photos in this smart album.
  And then delete both old keywords in the edit mode of the keyword manager.
  It is no problem to create a temporary smart album with "File > new smart album" for the search and to delete it when you are done. I keep one smart album that I always use for searching.
  
  If you are using iCloud Photo Library this may result in a very long period of syncing, if you have to change the keyword in many photos.