Why can't I replace a field in a mongoose document?

Question

MongoDB

vk_account.save(function(err, vk_account) {
  if(err) return cb(err, null);
  vk_account.Sources = updated_sources;
  vk_account.settings = account_setting;
  cb(null, vk_account);
})

In vk_account.settings(Should be an Array in mongoose schema terms) I assign account_setting(Object). As a result, absolutely nothing changes in vk_account.settings.
I don't want to go against the mongoose schema system, I'm not going to save this document, I just want to replace one field with another and give the client the result.