To put the Prime in diagnostic mode, press F C O keys all at the same time while doing a pin reset to back of the unit. It should display the ‘EA656 DIAGNOSTIC’ Screen Menu.
HP Prime reset to factory settings - Press 4 (FLS Utility), and then press 3 (Format Disk C). This clears out the flash. Once finished, press ESC to back out into the main menu. From there press 9 (RESET). It will reboot into its factory condition and request language choice.
Note: To do a quick minor factory reset. Press [Apps] and [Esc] key simultaneously and holding it, and press the [On] key.