gistfile1.txt

% mongo kk.js
MongoDB shell version v4.0.17
connecting to: mongodb://127.0.0.1:27017/?gssapiServiceName=mongodb
Implicit session: session { "id" : UUID("e953d631-6d34-4c7b-9fb3-2d99545b61a6") }
MongoDB server version: 4.0.17
BEFORE
{
	"_id" : ObjectId("5ec7e6bbce039a1491d73730"),
	"name" : "l33t shirt",
	"price" : "1200",
	"quantity" : "60",
	"options" : {
		"type" : "radio",
		"title" : "color",
		"opts" : [
			{
				"value" : "red",
				"price" : "-100",
				"qty" : 30
			},
			{
				"value" : "blue",
				"price" : "+200",
				"qty" : 10
			},
			{
				"value" : "green",
				"price" : "+300",
				"qty" : 20
			}
		]
	}
}
AFTER
{
	"_id" : ObjectId("5ec7e6bbce039a1491d73730"),
	"name" : "l33t shirt",
	"price" : "1200",
	"quantity" : "60",
	"options" : {
		"type" : "radio",
		"title" : "color",
		"opts" : [
			{
				"value" : "red",
				"price" : "-100",
				"qty" : 30
			},
			{
				"value" : "blue",
				"price" : "+200",
				"qty" : 9
			},
			{
				"value" : "green",
				"price" : "+300",
				"qty" : 20
			}
		]
	}
}

kk.js

db.test.remove({});

db.test.insert({
  name: "l33t shirt",
  price: "1200",
  quantity: "60",
  options: {
    type: "radio",
    title: "color",
    opts: [
      { value: "red" , price: "-100" , qty: 30  },
      { value: "blue" , price: "+200" , qty: 10  },
      { value: "green" , price: "+300" , qty: 20  }
    ]
  }
});

print("BEFORE");
printjson(db.test.findOne());

db.test.updateOne({ "options.opts.value": "blue" }, { $inc: { "options.opts.$.qty": -1 } });

print("AFTER");
printjson(db.test.findOne());