Backup Exports can be performed from the Export tab in Channels.
To create a new ItemMatrix Export, simply:
Click the Export Data tab
Select ItemMatrix from the dropdown
Select Export All or Export Data modified since a selected date
Press the Export button.
An export file in CSV format will be emailed to the email address on file soon thereafter.
The resulting file will include all fields available in the same format as the import template. This file can be imported into Excel and edited as needed, then reimported in whole or in part matching on MatrixID.
Warning: Opening the export file directly will result in Excel reformatting all number fields. Be sure to use the Import function from Excel's File menu and reformatted columns that hold large numbers to "Text".