To completely restore a Windows 10 IoT device to the Factory Default image, you need to download it from My Praim. The image is the standard factory image available on the out-of-the-box Praim devices, so once the device is restored, the entire disk will be completely rewritten and you will lose all the modifications made on the device.
From MyPraim, go to Products -> Downloads, use as filter the type: Image and download the Win10IoT image corresponding to the desired language. E.g. Win10IoT 4.2.0 GB.
Agile - Reset Configuration
Note that using the Agile - Reset the configuration of a device command will only affect the settings of the Agile environment (resources, write filter, remote assistance configuration and so on) and not all the Windows O.S. modifications that you eventually made.
You can write the image into the device in two possible ways: .1 Through the ThinMan Console using the Apply Device Image command (see Contextual Menu - Capture Image and Apply Image). Note that before calling this operation on the device you have to add the factory image into the ThinMan Package Repository. 2. Using the Praim Toolkit software (see Toolkit - How to create Recovery USB Key for Windows Embedded) and a USB Mass Storage key. Note that in this case the factory image file has to be available to the Praim Toolkit software, and it is the firmware file mentioned in the previous link.
Note that in both cases you have to start with the factory image file. For more information read the Backup, Restore and Device Cloning chapter.