Periodically, the CaptureApp will ask you to clear data from the SD card to help make room for new scan data. To delete old scans, navigate to the files folder from the home screen.
Then using the 3 lines in the top-left corner, open the menu and select the external disk.
The images are usually saved in the DCIM folder
Once in the folder, long-hold on the folder you wish to delete to select it and choose the trash-can icon in the upper right-hand corner to delete the folder.