Mongoose update variable with sum of old and new

I want to update new balance of user with summing old one using only update method.

users.update({user_id: 12345}, {balance: balanceFromFunc+oldBalance}); 

Are there any easy way to do this? Or do I have to do it like this? :

users.where(.... err, docs){
  var oldBalance = users.docs.balance ;
  var newBalance = "somevalue";
  var NEW_Balance = newBalance + oldBalance;
  users.update(...,balance : NEW_BALANCE)
}