Database
[{"_id":ObjectId("62792b4a0c9c5a00b6a8e17b"),"username":"user_1","words":[{"word":"RATIONAL","subwords":["RAT","TAN"]},{"word":"YOUNGER","subwords":["YOU","YOUR"]}]}]
Query
db.collection.update({},{$push:{"words.$[x].subwords":"AT"}},{arrayFilters:[{"x.word":"RATIONAL"}]})