how to update a map value by reflection ( params is interface )

Encountered such a problem, expect to make a general method to update the value in the map, as in the above example：

type Student struct {
    Name string `json:"name"`
}

func TestReflect(t *testing.T) {
    a := make(map[int]Student)
    update(&a)
    fmt.Println("outer:",a)
}

func update(a interface{}) {
    b := copyStruct(a)
    valueStr := "{\"0\":{\"name\":\"test\"}}"
    _ = json.Unmarshal([]byte(valueStr), &b)
    fmt.Println("inner:",b)
    a = b
}

func copyStruct(a interface{}) interface{} {
    // Create a pointer type to the underlying element type
    return reflect.New(reflect.TypeOf(a).Elem())
}

The expected output result is： inner: map[0:map[name:test]] outer: map[0:map[name:test]]

But the actual output is : inner: map[0:map[name:test]] outer: map[]

How can i do .... Can somebody help