upsertArray.js

// args
// questionId, selectedChoiceId

const question = await Question.findById(id);

// search if questionId is exist
if(question.answers.filter( answer => answer.questionId === questionId ).length === 0){
  question.answers.push({ questionId, selectedChoiceId });
} else {
  // find and update
  question.answers = question.answer.map((answer, index) => {
    if(answer.questionId === questionId) {
      answer.selectedChoiceId = selectedChoiceId;
    }
  });
}

await question.save();

// return response