Can I update an existing Amazon S3 object?

You can use Athena to update with extra rows via an INSERT function. You will need to provide Athena with access to the S3 bucket, crawl the S3 bucket using Amazon Glue to get the table layout, update Glue for the Schema names and data types. Once this has been done you can then update using the Athena console at https://console.aws.amazon.com/athena/home

INSERT INTO destination_table [(col1,col2,...)]
VALUES (col1value,col2value,...)[,(col1value,col2value,...)][,...]

Updating an existing object in Amazon S3 doesn't differ from creating it in the first place, i.e. the very same PUT Object operation is used to upload the object and will overwrite the existing one (if it isn't protected by other means, e.g. via Using Bucket Policies or Object Versioning)

You can find a complete code sample in Upload an Object Using the AWS SDK for Java, the main part boils down to:

AmazonS3 s3client = new AmazonS3Client(new PropertiesCredentials(
        S3Sample.class.getResourceAsStream(
                "AwsCredentials.properties")));
try {
    System.out.println("Uploading a new object to S3 from a file\n");
    File file = new File(uploadFileName);
    s3client.putObject(new PutObjectRequest(
                             bucketName, keyName, file));

 } catch (AmazonServiceException ase) {
    System.out.println("Caught an AmazonServiceException, which " +
            "means your request made it " +
            "to Amazon S3, but was rejected with an error response" +
            " for some reason.");
    // ... error handling based on exception details
}