[{"_id":{"$oid":"630f3c32c1a580642a9ff4a0"},"title":"This is a title","Post":"this is a post"},{"_id":{"$oid":"630f3c32c1a580642a9ff4a1"},"title":"This is a titleB","Post":"this is a post"}]

db.collection.update({},[{$set:{slug:{"$replaceAll":{input:{$toLower:"$title"},find:" ",replacement:"-"}}}}],{multi:true})