[{_id:1,filmName:"Film number 1",episodes:[{episodeName:"Testing 1",slugEpisode:"testing-1",_id:301},{episodeName:"Testing 2",slugEpisode:"testing-2",_id:302},],},{_id:2,filmName:"Film number 2",episodes:[{episodeName:"Something 1",slugEpisode:"something-1",_id:401},{episodeName:"Something 2",slugEpisode:"something-2",_id:402},],}]

db.collection.update({"_id":2},{$set:{"episodes.$[elem].episodeName":"UpdatedValue"}},{arrayFilters:[{"elem._id":402}]})