Please note: a hard-reset will completely erase all the data, settings and content on your smartphone, returning it to the original factory settings. We strongly advise that you back up your device before a hard-reset. Visit here to learn more about how to back up your smartphone remotely.
To reset your smartphone and erase all its content, first make sure your phone is connected to a power source. We also recommend that your battery is at least 50% charged when you perform a hard reset.
Press the volume up key and the power button simultaneously for about 30 seconds or until the root menu appears. Then choose the action you want to take from the root menu, in this case "Wipe data/factory reset".
Alternatively, you can perform a hard reset by tapping Settings > Backup & Reset > Factory data reset > Reset phone > Erase everything.