[{"id":1,"product":[{"name":"Book","price":100},{"name":"Pen","price":10}]}]

db.collection.update({"product.0.price":100},[{"$addFields":{"product":{"$map":{"input":"$product","as":"p","in":{"$mergeObjects":["$$p",{"$cond":{"if":{"$eq":["$$p.price",100]},"then":{"paid":"$$p.price"},"else":{}}}]}}}}}])