How to PUT an object in an array in MongoDB using Mongoose on form submission

My mongoDB schema looks like this:

var userSchema = mongoose.Schema({

local            : {
    email        : String,
    password     : String,
},
userInfo         : {
    fullname     : String,
    region       : String,
},
milesLog         : []

});

And I have the following HTML Form:

   <form action="/miles" method="put">
     <div class="form-group">
        <label>Miles</label>
        <input type="text" class="form-control" name="miles">
    </div>
    <button type="submit" class="btn btn-warning btn-lg">Submit</button>
  </form>

And I have following route:

  app.put('/miles', isLoggedIn, function(req, res) {

        // WHAT TO DO HERE
 });

Question

How to PUT an object containing the miles variable into the milesLog array within my user schema on form submission.

Here's an image of my MongoDB:

So after form submission my Database should look like this:

 {
"_id": {
    "$oid": "54eda3160fb053cc25a9e287"
},
"userInfo": {
    "region": "Europe - UK",
    "fullname": "max26"
},
"local": {
    "password": "$2a$08$xicDozPMtIiImhwUNuV6SO0llxEnHUK3VlzNh6G7OUgbJwfoxTECC",
    "email": "max@gmail.com"
},

"milesLog: [
     {"miles": "21"},
     {"miles": "55"}
],

"__v": 0
  }

Thank you for your Help.

Regards,