https://github.com/tidwall/sjson

package main

import "github.com/tidwall/sjson"

const json = `{"name":{"first":"Janet","last":"Prichard"},"age":47}`

func main() {
    value, _ := sjson.Set(json, "name.last", "Anderson")
    println(value)
}
{"name":{"first":"Janet","last":"Anderson"},"age":47}