This is the official page from which you can Download HTC Desire D820U Stock Firmware ROM (flash file) package on your Computer.
The firmware package comes as a compressed file (zip file) which contains Flash File, SP Flash Tool, USB Driver and How-to Flash Manual.
HTC Desire D820U Stock Firmware ROM (flash file)
How to Flash HTC Desire D820U Stock ROM
Step 1: Download and upzip the firmware on your computer.
Step 2: After unzipping the Firmware package, you will get the Firmware (ROM), SP Flash Tool and Driver.
Step 3: Install the provided USB Driver on your computer.
Step 4: Open the SP Flash Tool and Load the Firmware.
Step 5: Connect your HTC Desire D820U device to the computer.
Step 6: Click on the Download button to begin the Flashing process.
Step 7: Once flashing is done, disconnect the device from the computer and restart it.
[*] HTC Driver: If you want to download the latest version of HTC USB Driver, then head over to the HTC USB Driver Download Page.
[*] Take a Backup: Flashing the above Firmware will delete the personal data from the device and you wont be able to recover it in anyway. Therefore, we recommend you to take a backup of the personal data before flashing the above firmware.