change single character to Uppercase or lowercase (not unicode)

Uppercase-lowercase.go

package main

import (
	"bytes"
	"fmt"
	"unicode"
)

func main() {

	str := []byte("hello THIS is me ")

	output := bytes.Map(func(r rune) rune {
		switch {
		case r >= 'A' && r <= 'Z':
			return unicode.ToLower(r)
		default:
			return unicode.ToUpper(r)
		}
	}, str)

	fmt.Println(string(output))

}