Update embedded documents in MongoDB

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MongoDB

I have made a collection "post" just as a tester which for now includes 2 documents. It has an embedded collection "comments" where i have entered 2 docs. A comment has a 'title', 'content' and 'likes'. I would like to know how could I add another field which is a collection named 'replies'(Basically replies to the comments)?
Here is my insert code for the initial documents:

post=db.post
post.insert
(
	[
		{title:"My first Post",
		content:"Hello World this is my first post 		at the website!!!",
		time: new Date(),
		by:{
			name: "Mohit Hiralal",
			email:"mh@gmail.com"}
		},
		{title:"My second post",
		content:"This is my second post. Hope this garners some atention!",
		time: new Date(),
		by{
name:"Mohit",email:"m@gmail.com"
		},
		comments:
		[
			{title:"Spelling wrong",
			content:"You spelled attenttion wrong…",
			likes:1009}
		]
		}
]
)