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How to update content of a text file?

In below example first character of a text file updated using WriteAt function which writes len(b) bytes to the File starting at byte offset off. It returns the number of bytes written and an error, if any.
"Po" has been updated by "Go" in below example.
package main

import (
	"io/ioutil"
	"log"
	"fmt"
	"os"
)

func Beginning() {
	// Read Write Mode
	file, err := os.OpenFile("test.txt", os.O_RDWR, 0644)
	
	if err != nil {
		log.Fatalf("failed opening file: %s", err)
	}
	defer file.Close()
	
	len, err := file.WriteAt([]byte{'G'}, 0) // Write at 0 beginning
	if err != nil {
		log.Fatalf("failed writing to file: %s", err)
	}
	fmt.Printf("\nLength: %d bytes", len)
	fmt.Printf("\nFile Name: %s", file.Name())
}

func ReadFile() {
	data, err := ioutil.ReadFile("test.txt")
	if err != nil {
		log.Panicf("failed reading data from file: %s", err)
	}
	fmt.Printf("\nFile Content: %s", data)	
}

func main() {
	fmt.Printf("\n######## Read file #########\n")
	ReadFile()
	
	fmt.Printf("\n\n######## WriteAt #########\n")
	Beginning()
	
	fmt.Printf("\n\n######## Read file #########\n")
	ReadFile()
}

C:\golang\codes>go run example16.go

######## Read file #########

File Content: Po Programming Language

######## WriteAt #########

Length: 1 bytes
File Name: test.txt

######## Read file #########

File Content: Go Programming Language
C:\golang\codes>