@Test
    public void howToUseUpdateExpression() {
        Table table = dynamo.getTable(TABLE_NAME);
        table.updateItem(HASH_KEY, FIRST_CUSTOMER_ID, RANGE_KEY, ADDRESS_TYPE_WORK,
            // update expression
            "set #phoneAttributeName = :phoneAtributeValue", 
            new NameMap().with("#phoneAttributeName", "phone"),
            new ValueMap().withStringSet(":phoneAtributeValue",
                "123-456-7890", "987-654-3210")
            );
        GetItemOutcome outcome = table.getItemOutcome(new GetItemSpec()
            .withPrimaryKey(HASH_KEY, FIRST_CUSTOMER_ID, RANGE_KEY, ADDRESS_TYPE_WORK)
            .withConsistentRead(true));
        Item item = outcome.getItem();
        Set<String> phoneNumbers = item.getStringSet("phone");
        assertTrue(phoneNumbers.size() == 2);
        System.out.println(phoneNumbers);
    }
Sample code to update items to a dynamo table.
src/samples/AmazonDynamoDBDocumentAPI/quick-start/com/amazonaws/services/dynamodbv2/document/quickstart/F_UpdateItemTest.java