In the new window, tap on Factory data reset and click on reset phone at the bottom of the screen and enter the password if you set any to your device. (phoneradar.com)
To do this, open the Settings screen, tap Backup & reset, tap Factory data reset, and follow the instructions on your screen. (howtogeek.com)
Now give an account where all the data has to be backed up and to select the option factory data to reset. (phoneradar.com)