Update field in exact element array in MongoDB

Try update document in array using positional $,

The positional $ operator facilitates updates to arrays that contain embedded documents. Use the positional $ operator to access the fields in the embedded documents with the dot notation on the $ operator.

db.collection.update(
  { "heroes.nickname": "test" },
  { $set: { "heroes.$.items.1": "new_value" } },
  { multi: true }
);

Playground

You need to make use of 2 concepts: mongodb's positional operator and simply using the numeric index for the entry you want to update.

The positional operator allows you to use a condition like this:

{"heroes.nickname": "test"}

and then reference the found array entry like so:

{"heroes.$  // <- the dollar represents the first matching array key index

As you want to update the 2nd array entry in "items", and array keys are 0 indexed - that's the key 1.

So:

> db.denis.insert({_id:"43434", heroes : [{ nickname : "test",  items : ["", "", ""] }, { nickname : "test2", items : ["", "", ""] }]});
> db.denis.update(
    {"heroes.nickname": "test"}, 
    {$set: {
        "heroes.$.items.1": "new_value"
    }}
)
> db.denis.find()
{
    "_id" : "43434", 
    "heroes" : [
        {"nickname" : "test", "items" : ["", "new_value", "" ]},
        {"nickname" : "test2", "items" : ["", "", "" ]}
    ]
}