How to append to a .go file

Getting Help
1

Hello i want to append new lines where i want in a .go file for example
i want to append fmt.Println(“new line”) before fmt.Println("Hello, world)
and also i want to add new line after “fmt” for example “myPackage”,

Does anyone have a example?

package main

import (
  "fmt"
 //"myPackage"
)

func main() {
    //fmt.Println("new line")
    fmt.Println("Hello, world)
}