如何从地图界面解析特定键？

I am reading in a yaml file, and unmarshaling it into a map[interface{}]interface{}. The goal is to replace a certain value within the yaml, and update the file read in with just that value replaced. I'm not sure how to parse the map though to get to the specific key (password) I want to change. I believe you can use reflect, but I'm not 100% sure of the syntax

YAML file I am reading in (config.yml):

i_name: example

instances:
  - name: test-instance
    command: get
    arguments:
        hostname: localhost
        port: 203
        username: test
        password: testing123
    labels:
        env: dev

Code thus far:

func replaceConfigPassword() {
    yamlFile, err := ioutil.ReadFile("config.yml")
    fatalIfErr(err)

    m := make(map[interface{}]interface{})

    err = yaml.Unmarshal(yamlFile, &m)
    fatalIfErr(err)

    fmt.Println(m)

    val := reflect.ValueOf(m["instances"]["arguments"]["password"])
    fmt.Println("val=", val)

The following shows how you might step through the yaml to arrive at the password:

package main

import (
    "fmt"
    "gopkg.in/yaml.v2"
)

func main() {
    in := []byte(`
i_name: example

instances:
  - name: test-instance
    command: get
    arguments:
        hostname: localhost
        port: 203
        username: test
        password: testing123
    labels:
        env: dev
`)

    m := make(map[string]interface{})
    if err := yaml.Unmarshal(in, &m); err != nil {
        panic(err)
    }
    fmt.Printf("%+v
", m)

    instances := m["instances"].([]interface{})
    fmt.Printf("%+v
", instances)

    first := instances[0]
    fmt.Printf("%+v
", first)

    arguments := first.(map[interface{}]interface{})["arguments"]
    fmt.Printf("%+v
", arguments)

    password := arguments.(map[interface{}]interface{})["password"]
    fmt.Printf("%+v
", password)
}

$ go run main.go
map[i_name:example instances:[map[name:test-instance command:get arguments:map[password:testing123 hostname:localhost port:203 username:test] labels:map[env:dev]]]]
[map[name:test-instance command:get arguments:map[hostname:localhost port:203 username:test password:testing123] labels:map[env:dev]]]
map[name:test-instance command:get arguments:map[hostname:localhost port:203 username:test password:testing123] labels:map[env:dev]]
map[username:test password:testing123 hostname:localhost port:203]
testing123

If you know the input structure before hand, another option could be to model it explicitly:

type Config struct {
    IName string `yaml:"i_name"`
    Instances []struct{
        Name string
        Command string
        Arguments map[string]interface{}
        Labels map[string]string
    }
}

    var c Config
    if err := yaml.Unmarshal(in, &c); err != nil {
        panic(err)
    }
    fmt.Printf("%+v
", c)
    fmt.Printf("%+v
", c.Instances[0].Arguments["password"])

$ go run main.go

{IName:example Instances:[{Name:test-instance Command:get Arguments:map[hostname:localhost port:203 username:test password:testing123] Labels:map[env:dev]}]}
testing123