Updating value of a subfield in MongoDB with Java driver?

I've quite new to MongoDB and it's Java driver.
I need to update the value of a subfield, but I can't find any examples online.

The document:

{
    "_id" : ObjectId("45678942342"),
    "user" : "me",
    "aStruct" : {
        "subfield_1" : true,
        "subfield_2" : true
    }
}

How do I update the value of subfield subfield_1 to false, for every document that has user = me ?

Thank you.