Software

There are two ways to update...over the air using a good strong WiFi network or by using iTunes on a computer. You cannot use cellular data.

Take it to an Apple Store and see then to update it for you, or go to a place with public WiFi, or a friend or family member's house that has WiFi or a computer running iTunes.

Update your iPhone, iPad, or iPod touch - Apple Support

Hi,

You need a wifi to update. Try to connect to hotspot of other mobiles of your family members or friends and do the update. Make sure your battery charge more than 50% during the update.