Can AWS Athena update or insert data stored in S3?

Amazon Athena is, indeed, a query service -- it only allows data to be read from Amazon S3.

One exception, however, is that the results of the query are automatically written to S3. You could, therefore, use a query to generate results that could be used by something else. It's not quite updating data but it is generating data.

My previous attempts to use Athena output in another Athena query didn't work due to problems with the automatically-generated header, but there might be some workarounds available.

If you are seeking a service that can update information in S3, you could use Amazon EMR, which is basically a managed Hadoop cluster. Very powerful and capable, and can most certainly update information in S3, but it is rather complex to learn.

Amazon Athena adds support for inserting data into a table using the results of a SELECT query or using a provided set of values

Amazon Athena now supports inserting new data to an existing table using the INSERT INTO statement.

https://aws.amazon.com/about-aws/whats-new/2019/09/amazon-athena-adds-support-inserting-data-into-table-results-of-select-query/

https://docs.aws.amazon.com/athena/latest/ug/insert-into.html

Bucketed tables not supported

INSERT INTO is not supported on bucketed tables. For more information, see Bucketing vs Partitioning.