Modifying JSON file using Golang

I am attempting to read in a JSON file in Golang, modifying this JSON file, and then creating a new JSON file/writing over this JSON file. I have seen several samples online, but can't seem to put two and two together to get the required result. I tried just making my own JSON str in GO and modifying that, but have still failed.

package main

import (
	"encoding/json"
	"fmt"
)

type Person struct {
	Name    string
	Age     int
	Details interface{}
}

func main() {
	//I created a simple Json structure here to play with
	str := `{"name": "A",
    	"age":20,
    	"details": {"salary":100000000}
    }`
	var data Person

	err := json.Unmarshal([]byte(str), &data)
	if err != nil {
		panic(err)
	}

	//I make a map so that I can adjust the value of the salary
	details, ok := data.Details.(map[string]interface{})
	if ok {
		details["salary"] = 999999
	}
	//Change the other values of a Person
	data.Name = "B"
	data.Age = 19
	fmt.Println(data)

	//SAMPLE OUTPUT: {B 19 map[salary:999999]}
}

I have made several attempts with reading the file, below is my best attempt:

package main

import (
	"encoding/json"
	"fmt"
	"io/ioutil"
	"os"
)

/* Create the structure that follows the outline of the JSON file*/

type UserType struct {
	User []struct {
		CdbID     string `json:"cdb_id"`
		Firstname string `json:"firstname"`
		Lastname  string `json:"lastname"`
		Phone     int64  `json:"phone"`
		Email     string `json:"email"`
		Address   []struct {
			Street  string `json:"street"`
			City    string `json:"city"`
			Zip     string `json:"zip"`
			Country string `json:"country"`
		} `json:"address"`
		Authenticators []struct {
			Name  string `json:"name"`
			Phone int64  `json:"phone"`
		} `json:"authenticators"`
		VoiceSig            string `json:"voice_sig"`
		VoicesigCreatedTime string `json:"voicesig_created_time"`
		Status              string `json:"status"`
	} `json:"user"`
}

func main() {
	file, err := ioutil.ReadFile("user.json") //Read File
	if err != nil {
		fmt.Print("Error:", err)
	}

	var u UserType
	json.Unmarshal(file, &u)     //Parse the Json-encoded Data and store the results in u
	result, e := json.Marshal(u) //Returns the Json encoding of u into the variable result
	if e != nil {
		fmt.Println("error", err)
	}
	os.Stdout.Write(result) //The line of code golang.org uses to print  the Json encoding
}

This is a sample output:

{
	"user": [{
		"cdb_id":"",
		"firstname":"Tom",
		"lastname":"Bradley",
		"phone":14155555555,
		"email":"tom@gmail.com",
		"address":[{
			"street":"4343 shoemaker ave",
			"city":"Brea",
			"zip":"92821",
			"country":"USA"
		}],
		"authenticators":[{
			"name":"Lisa Hayden",
			"phone":15625555555
		},{
			"name":"Pavan M",
			"phone":17145555555
		}],
		"voice_sig":"242y5-4546-555kk54-437879ek545",
		"voicesig_created_time":"2017-08-02T21:27:44+0000",
		"status":"verified"
	}]
}

I am just confused on how to modify what I want, specifically the "Authenticators" of the aforementioned sample output. Thanks!

Add a declaration

type Authenticator struct {
    Name  string `json:"name"`
    Phone int64  `json:"phone"`
}

and change

Authenticators []struct {
        Name  string `json:"name"`
        Phone int64  `json:"phone"`
} `json:"authenticators"`

to

Authenticators []Authenticator `json:"authenticators"`

Then, to add an authenticator to the first user:

u.User[0].Authenticators = append(u.User[0].Authenticators, Authenticator{
        Name: "John Doe",
        Phone: 1234567890,
})