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Upsert mutations allow you to perform add or update operations based on whether a particular ID exists in the database. The IDs must be external IDs, defined using the @id directive in the schema. For example, to demonstrate how upserts work in GraphQL, take the following schema:

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Schema

type Author {
  id: String! @id
  name: String! @search(by: [hash])
  posts: [Post] @hasInverse(field: author)
}

type Post {
  postID: String! @id
  title: String! @search(by: [term, fulltext])
  text: String @search(by: [fulltext, term])
  author: Author!
}
Dgraph automatically generates input and return types in the schema for the add mutation, as shown below: 
addPost(input: [AddPostInput!]!, upsert: Boolean): AddPostPayload

input AddPostInput {
  postID: String!
  title: String!
  text: String
  author: AuthorRef!
}
Suppose you want to update the text field of a post with the ID mm2. But you also want to create a new post with that ID in case it doesn’t already exist. To do this, you use the addPost mutation, but with an additional input variable upsert. This is a Boolean variable. Setting it to true results in an upsert operation. It performs an update mutation and carry out the changes you specify in your request if the particular ID exists. Otherwise, it falls back to a default add operation and create a new Post with that ID and the details you provide. Setting upsert to false is the same as using a plain add operation—it’ll either fail or succeed, depending on whether the ID exists or not. Example: add mutation with upsert true: 
mutation ($post: [AddPostInput!]!) {
  addPost(input: $post, upsert: true) {
    post {
      postID
      title
      text
      author {
        id
      }
    }
  }
}
With variables: 
{
  "post": {
    "postID": "mm2",
    "title": "Second Post",
    "text": "This is my second post, and updated with some new information.",
    "author": {
      "id": "micky"
    }
  }
}
If a post with the ID mm2 exists, it updates the post with the new details. Otherwise, it’ll create a new Post with that ID and the values you provided. In either case, you’ll get the following response back: 
"data": {
    "addPost": {
      "post": [
        {
          "postID": "mm2",
          "title": "Second Post",
          "text": "This is my second post, and updated with some new information.",
          "author": {
            "id": "micky"
          }
        }
      ]
    }
  }
The default value of upsert is false.
The current behavior of Add and Update mutations is such that they don’t update deep level nodes. So Add mutations with upsert set to true only updates values at the root level.

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Examples

You can refer to the following link for more examples.