To do what you want, you wound need to:

1. read JSON data from the file with fs.readFile()
2. parse the JSON with JSON.parse()
3. find the correct object in the array
4. change the object
5. serialize the object back to JSON with JSON.stringify()
6. write the file with fs.writeFile()

but this is not that simple as it may look like, because you will have to:

• add locking to writes so that you never do two writes at the same time
• add locking to reads so that you never read while the write is in progress
• handle incorrect JSON
• handle cases of objects that cannot be serialized
• avoid blocking operations (those with "Sync" in their name) anywhere else then in the first tick of the event loop

Considering all of that you should consider using a database to store any data that changes. Some databases like Mongo, Postgres or Redis need to be run as standalone application either on the same or on a different server. Some embedded databases like SQLite don't need a standalone process and can be run directly in your application.

It's not that it is impossible to write to JSON files and then read those files as needed, but the amount of work that you'd have to do to synchronize the access to the data all without accidentally blocking the event loop in the process is much more difficult than just using any database as intended.

You have some data:

const data = [
  {
    "Pref":"Freedom",
    "ID":"5545"
  },
  {
    "Pref":"Growth",
    "ID":"8946545"
  }
]

First we need to find the element you want to change (use [0] to only select the first in case there are multiple items with ID 5545:

const objectToChange = data.filter(item => item.ID === "5545")[0]

And then change it!

objectToChange['Pref'] = "Liberty"

We can see the change reflected in the data object:

console.log(data)

//  [{
//    ID: "5545",
//    Pref: "Liberty"
//  },{
//    ID: "8946545",
//    Pref: "Growth"
//  }]

1- Load file: let json = JSON.parse(fs.readFileSync('file.json', 'utf-8'));

2- Update content:

json = json.map(el => {
  if(el.ID === "5545") {
    el.Pref = "TEST";
  }
  return el;
});

3- Save again maybe?

fs.writeFileSync('test.json', JSON.stringify(json), 'utf-8');