How to download software update and install 15.7

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

After installing iOS update my iPhone 7s now goes too no service ( no cellular data ) and stays this way for 10 hours then it comes back on but today is day 6 and now it won’t come back on at all