Database

[{"_id":"4faaba123412d654fe83hg876","user_id":123456,"total":100,"items":[{"item_name":"my_item_one","price":20},{"item_name":"my_item_two","price":50},{"item_name":"my_item_three","price":30}]}]

Query

db.collection.update({user_id:123456,"items.item_name":"my_item_two",},{"$set":{"items.$.price":2}})

Result