System.Text.Json.deserialize Json String

Don't know why you want to use the JsonNode.Parse().

If you just want to modify the location's value, maybe you can use the JsonSerializer with BookStore and Book class to help you.
📌 The Book class is a IEnumerable property of BookStore.

an example by .NET6 Console App
📌 BookClass is a 2-level nested class

// See https://aka.ms/new-console-template for more information
using System.Text.Json;

// Initialize json string
string jsonStr = "{\"id\": 1, \"ShopName\" :\"Paint Shop\", \"Books\" : [\r\n            {\r\n                \"bookId\":1,\r\n                \"bookName\":\"Peter Pan\",\r\n                \"location\":\"A01\"\r\n            },\r\n            {\r\n                \"bookId\":2,\r\n                \"bookName\":\"Cooking Book\",\r\n                \"location\":\"A02\"\r\n            }\r\n        ]\r\n    }";

// Deserilize to object
Bookstore bookstore = JsonSerializer.Deserialize<Bookstore>(jsonStr);

// Write the location's value
Console.WriteLine(bookstore.Books.ToArray()[1].location);

// Modified the location's value
bookstore.Books.ToArray()[1].location += "_Modified";

// Write the modified location's value
Console.WriteLine(bookstore.Books.ToArray()[1].location);

// See result output
Console.ReadLine();

public class Bookstore
{
    public int id { get; set; }
    public string ShopName { get; set; }
    public IEnumerable<Book> Books { get; set; }
}

public class Book
{
    public int bookId { get; set; }
    public string bookName { get; set; }
    public string location { get; set; }
}

Result output image

If you persist to use JsonNode.Parse(), then need more steps. like this.

// See https://aka.ms/new-console-template for more information
using System.Text.Json;
using System.Text.Json.Nodes;

// Initialize json string
string jsonStr = "{\"id\": 1, \"ShopName\" :\"Paint Shop\", \"Books\" : [\r\n            {\r\n                \"bookId\":1,\r\n                \"bookName\":\"Peter Pan\",\r\n                \"location\":\"A01\"\r\n            },\r\n            {\r\n                \"bookId\":2,\r\n                \"bookName\":\"Cooking Book\",\r\n                \"location\":\"A02\"\r\n            }\r\n        ]\r\n    }";

// Parse to jsonNode
JsonNode jsonNode = JsonNode.Parse(jsonStr);

// Convert to JsonObject
JsonObject jsonObject = jsonNode.AsObject();

// Convert Books to Json Array
JsonArray jsonArray = jsonObject["Books"].AsArray();

// Convert index 1 in array to Json object
JsonObject book1Object = jsonArray[1].AsObject();

// Write the location value 
Console.WriteLine(book1Object["location"]);

// Modify location value 
book1Object["location"] += "_modified";

// Write the modified location value 
Console.WriteLine(book1Object["location"]);

// See output
Console.ReadLine();

public class Bookstore
{
    public int id { get; set; }
    public string ShopName { get; set; }
    public IEnumerable<Book> Books { get; set; }
}

public class Book
{
    public int bookId { get; set; }
    public string bookName { get; set; }
    public string location { get; set; }
}

Other suggestion

Use Newtonsoft.Json is better. Because when convert a json to a C# object, will need to convert many properties with different value type. The Newtonsoft.Json had already handle these situation, at the mean time the System.Text.Json does not.

You can use JObject to modify json directly

var jObject = JObject.Parse(json);

jObject["Books"][1]["location"] = "new value";

Result:

Note:

You should add Newtonsoft.Json package.

Since you are using System.Text.Json, you can change value using this code

var jsonParsed=JsonNode.Parse(json);
jsonParsed["Books"][1]["location"]="A03";

after this , if you need you can get an updated json

json = System.Text.Json.JsonSerializer.Serialize(jsonParsed, new JsonSerializerOptions {WriteIndented=true});