How to change some bytes in a byte[] at a specific index in GO programming?

I have a []byte that which is essentially a string, in this array I have found something I want to change using index:

content []byte
key []byte
newKey []byte
i = bytes.Index(content, key)

So I have found key in content (at index I), now I want to replace key with newKey but I can't seem to find a way to add it in I was trying the obvious thing that wouldn't work

content[i] = newKey

Is there some function that allows me to replace "key" with "newKey" within content []byte?

Thanks,

Following the article "Go Slices: usage and internals", you can use copy in order to create a slice with the right content:

playground

package main

import "fmt"

func main() {
	slice := make([]byte, 10)
	copy(slice[2:], "a")
	copy(slice[3:], "b")
	fmt.Printf("%v\n", slice)
}

Output:

[0 0 97 98 0 0 0 0 0 0]

In your case, if len(key) == len(newJey):

playground

package main

import "fmt"
import "bytes"

func main() {
	content := make([]byte, 10)
	copy(content[2:], "abcd")
	key := []byte("bc")
	newKey := []byte("xy")
	fmt.Printf("%v %v\n", content, key)
	
	i := bytes.Index(content, key)
	copy(content[i:], newKey)
	fmt.Printf("%v %v\n", content, newKey)
}

Output:

[0 0 97 98 99 100 0 0 0 0] [98 99]
[0 0 97 120 121 100 0 0 0 0] [120 121]