How to forcibly flush the undo log/tablespace of UNDOTBS1
Undo data is used to ensure ACID of the database. It supports your transaction and cannot be removed until you committed or rolled it back. Read "What Is Undo?" in Database Administrator's Guide.

You can however commit your transaction to release some undo space, but you lose the ability to roll back your changes.

If the DML you are performing requires more undo space because there are a lot of records updated/deleted/inserted and your undo tablespace is not sized correctly, DML will fail, because Oracle cannot allocate extents to support your transaction.