This is the official page from which you can Download HTC U11 Stock Firmware ROM (flash file) package on your Computer.
The firmware package comes as a compressed file (zip file) which contains Flash File, Flash Tool, USB Driver and How-to Flash Manual.
HTC U11 Stock Firmware ROM (flash file)
Package Name: HTC_U11_RUU_OCEAN_WHL_O80_SENSE90GP_MR_NA_Gen_Unlock_2.42.617.7_EXE.zip
Package Size: 2 GB
How-to Flash: Open and exe file from firmware folder and follow the screen instructions.
Firmware Link: Click Here to Get the Firmware
How to Flash HTC U11 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), Flash Tool and Driver.
Step 3: Install the provided USB Driver on your computer.
Step 4: Open the Firmware Folder and launch the .exe file.
Step 5: Connect your HTC U11 device to the computer.
Step 6: Click on the Next 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.