[{"_id":1,"myarray":[{"id":1,"qty":1},{"id":2,"qty":2}]},{"_id":2,"myarray":[{"id":1,"qty":1},{"id":2,"qty":2}]}]

db.collection.update({"_id":{"$eq":1}},[{"$addFields":{"myarray":{"$map":{"input":"$myarray","as":"d","in":{"$mergeObjects":["$$d",{"qty":{"$add":["$$d.qty",10]}}]}}}}}])