These are the instructions to unlock the bootloader of HTC One A9 using the official unlock method.
Caution: Unlocking the bootloader will delete your personal data from the device. So, backup everything before proceeding.
How to Unlock HTC One A9 Bootloader (GSM)
Step 1. Download and install HTC USB Driver on your computer. In case you have already installed the HTC Driver on your computer then skip this step.
Step 2. Open HTC Dev Official Page and Register a New account. Once registration is completed you will receive activation email, you need to open the activation link from the email in order to activate your HTC Dev account.
Step 3. Once you have successfully activated the HTC dev account, you can login to HTC Dev account. After logging into HTC Dev account Open Unlock Bootloader page.
Step 4. On Unlock Bootloader page find Supported Devices section on the left side. Under Supported Devices Select “All other supported models”. After that click on the Green Begin Unlock Bootloader Button.
Step 5. Now you will be able to see pop-up confirmation message “Are you sure you wish to continue?“, click on Yes to continue the unlocking process.
Step 6. Again you will see a pop-up which prompts to accept legal terms. Tick both boxes and click on the Proceed to Unlock Instructions button.
Step 7. On the next page you will be able to see instructions to enter into Bootloader mode on your Phone. You have to follow the instructions to enter into Bootloader.
To Enter into Bootloader Mode:
a. Switch off your Phone.
b. Press and Hold Volume Down + Power Button until you enter into the Bootloader mode.
Step 8. Once you enter into Bootloader Mode, Press Power Button once. After pressing the Power Button you will enter into the FastBoot mode.
Step 9. Now, connect your HTC One A9 to the computer.
Step 10. Download and install Minimal ADB and Fastboot Tools on your computer.
Step 13. In the command window, give the following command: fastboot oem get_identifier_token
Step 14. Now, you will be able to see the identifier token of your HTC One A9. You have to copy the whole token, to copy right click on the CMD and Select Mark.
Now, Select the whole identifier token as shown in the below picture:
Once you have selected the identifier token, Left click the Mouse Button to Copy.
 Do not close the command prompt.
Step 15. Now again open the HTC Dev Official website and enter the copied identifier token (Step #10 page). Once you have pasted the identifier token, click on Submit button.
Step 16. Now, you will get an email from HTC Dev team with the attached Unlock_code.bin file. You have to download this file and place it in the “root” folder that you have created in Step #11.
Step 17. Once you have placed the Unlock_code.bin file in root folder, Now again come back to command prompt to give another command.
Command: fastboot flash unlocktoken Unlock_code.bin
Step 18. When unlocking is completed your phone will restart automatically.
 Data loss Process: Unlocking process will delete your phone data like contacts, messages, pictures, music, videos, applications and games.
 Unlock Other HTC Device Bootloader: If you want to Unlock the bootloader of any other HTC Device then head over to the How to unlock HTC Bootloader page.
 HTC Stock Firmware: If you are looking for the original firmware for your device, then head over to Download HTC Stock Firmware page.
 HTC USB Driver: If you are looking for the original USB Driver for your device, then head over to Download HTC USB Driver page.