func main() {

	d := bson.D{bson.DocElem{"name", "bob"}}

	fmt.Print(d.Map()["name"])

	updateBsonDField(d, "name", "sam")

	fmt.Print(d.Map()["name"])
}

func updateBsonDField(d bson.D, name string, value interface{}) error {
	for _, field := range d {
		if field.Name == name {
			field.Value = value
			return nil
		}
	}
	return errors.Errorf("field %q not found", name)
}

---- output ----
bobbob