code to achieve this

/*
##################################################################################
##################################################################################
######### IF YOU FOUND THIS GIST USEFUL, PLEASE LEAVE A STAR. THANKS. ############
##################################################################################
##################################################################################
*/    
    
    String content = FileUtils.readFileToString(new File("src/main/resources/sample.json"), "UTF-8");
    DocumentContext jsonContext = JsonPath.parse(content);
    jsonContext.delete("$.reservations[*].customers");
    jsonContext.renameKey("$.reservations[*]","id","reservations.id");
    System.out.println(jsonContext.jsonString());

dependency

<dependency>
  <groupId>com.jayway.jsonpath</groupId>
  <artifactId>json-path</artifactId>
  <version>2.4.0</version>
</dependency>

input json

{
   "reservations":[
      {
         "id":"1318504",
         "deposit":720,
         "rooms":[
            {
               "name":"room1",
               "id":"28902"
            },
            {
               "name":"room2",
               "id":"28906"
            }
         ],
         "customers":[
            {
               "id":3,
               "country":"UK",
               "firstName":"John"
            }
         ]
      },
      {
         "id":"1318501",
         "deposit":68,
         "rooms":[
            {
               "name":"room3",
               "id":"28860"
            },
            {
               "name":"room4",
               "id":"28886"
            }
         ]
      }
   ]
}

Json Manipulation using JsonPath.md

A simple example to remove/manipulate json nodes is by using JsonPath

output json

{
   "reservations":[
      {
         "deposit":720,
         "rooms":[
            {
               "name":"room1",
               "id":"28902"
            },
            {
               "name":"room2",
               "id":"28906"
            }
         ],
         "reservations.id":"1318504"
      },
      {
         "deposit":68,
         "rooms":[
            {
               "name":"room3",
               "id":"28860"
            },
            {
               "name":"room4",
               "id":"28886"
            }
         ],
         "reservations.id":"1318501"
      }
   ]
}