Reset to factory settings for Microsoft Lumia 540 is performed as standard for the series Lumia:
Turn off the phone
Hold down the "power"
After activation of the vibration, release the button "start" and quickly press and hold "Volume -" until the appearance of an exclamation mark
Click "Volume +" "Volume -" , "Enabling" and then "Volume -"
The phone will reboot and on the screen you will see the gears, indicating a well-executed combination. After loading the phone settings will be reset to the factory default and will erase all user information.