When your customers buy your product, an order is generated in Zakeke as well, you may need to modify the print-ready files for the orders your receive.
After clicking on Edit Print-Ready Files, Zakeke will display the customizer screen and it will be possible to proceed with editing the design:
Once you're done with the editing, click on Update Print Files to save the new design.
IMPORTANT: new print-ready files override the previous print-ready files and you will no longer able to recover them.