You have several options to backup or export your data from Bkper.
Export your data to a CSV file with the content according to a query.
Extract your data to a Google Sheet, using the Bkper add-on. On the add-on select your book, query and combination of data to extract, press fetch and Bkper data will be written from the active cell on.
Note: The add-on auto update fetches the data every hour for you, ideal for automated backups.
Build your own extraction with the BkperApp library, It gives you the freedom to export your data where you want it, using Google Apps Script.