how to restore iPhone 7 to factory default

After update in itune my iPhone 7 can't turn on

If the iPhone 7 will not power on, then there is no way to restore factory defaults to the iPhone. So, your first task will be to try to get the iPhone to power up.

Perform a Forced Restart on the phone. Follow the instructions very carefully, as this is a bit tricky. You may need to repeat this several times.

Force restart iPhone - Apple Support

There are only 2 possible results when you perform a Forced Restart:

1) The phone returns to normal operation

or.....

2) There is a hardware problem with the phone that will need attention at the Apple Store (make an appointment) or at an authorized Apple Service shop

You can find an authorized service location by using this link: Find Locations

See -> Restore your iPhone, iPad, or iPod to factory settings

Restore your iPhone, iPad, or iPod to factory settings - Apple Support