code: public TransactionWriteRequest addUpdate(Object object,
                                               DynamoDBTransactionWriteExpression transactionWriteExpression,
                                               ReturnValuesOnConditionCheckFailure returnValuesOnConditionCheckFailure) {
          transactionWriteOperations.add(new TransactionWriteOperation(object, TransactionWriteOperationType.Update, transactionWriteExpression, returnValuesOnConditionCheckFailure));
          return this;
      }
docs: Adds update operation (to be executed on object) to the list of transaction write operations.
  transactionWriteExpression is used to conditionally update object.
  returnValuesOnConditionCheckFailure specifies which attributes values (of existing item) should be returned if condition check fails.