String

In Go, string is an immutable array of bytes. So if created, we can't change its value. E.g.:

package main

func main()  {
    s := "Hello"
    s[0] = 'h'
}

The compiler will complain:

cannot assign to s[0] 

To modify the content of a string, you could convert it to a byte array. But in fact, you do not operate on the original string, just a copy:

package main

import "fmt"

func main()  {
    s := "Hello"
    b := []byte(s)
    b[0] = 'h'
    fmt.Printf("%s\n", b)
} 

The result is like this:

hello

Since Go uses UTF-8 encoding, you must remember the len function will return the string's byte number, not character number:

package main

import "fmt"

func main()  {
    s := "日志log"
    fmt.Println(len(s))
} 

The result is:

9

Because each Chinese character occupied 3 bytes, s in the above example contains 5 characters and 9 bytes.

If you want to access every character, for ... range loop can give a help:

package main
import "fmt"

func main() {
    s := "日志log"
    for index, runeValue := range s {
        fmt.Printf("%#U starts at byte position %d\n", runeValue, index)
    }
}

The result is:

U+65E5 '日' starts at byte position 0
U+5FD7 '志' starts at byte position 3
U+006C 'l' starts at byte position 6
U+006F 'o' starts at byte position 7
U+0067 'g' starts at byte position 8

Reference:
Strings, bytes, runes and characters in Go;
The Go Programming Language.