HTC One User Manual page 185

Files such as your music, photos, videos, and documents will also be
deleted if you select the Erase all data option.
Be sure to back up any data and files you want to keep before you do a
factory reset.
A factory reset may not permanently erase all data from your
phone, including personal information.
Performing a factory reset from settings
1. Go to Settings, and then tap Backup & reset.
2. Tap Reset phone.
3. If you also want to delete media and other data on HTC One, select
Erase all data.
4. Tap OK.
Performing a factory reset using hardware buttons
If you can't turn HTC One on or access settings, you can still perform a
factory reset by using the hardware buttons on HTC One.
1. Press and hold the VOLUME DOWN button, and then press and
hold the POWER button.
2. Wait for the screen with the three Android images to appear, and
then release the POWER and VOLUME DOWN buttons.
3. Press VOLUME DOWN to select FACTORY RESET, and then press
the POWER button.
Sync, backup, and reset 185