Why won't my iPhone update ?

First do a Forced restart Force restart iPhone – Apple Support (UK)

then

If iPhone will not update

Check all of these carefully:-

·       you are on a good Wi-Fi, (from 15.4 iPhone 7 up, you may be offered cellular update)

·       Plenty of spare storage – 6GB at least.

·       on charge with more than 20%, OR 

·       off charge with more than 50%

·       low power mode turned off. 

·       Check Settings, General to make sure you have no Beta profile installed.

·       no VPN installed (Check Settings, General, VPN & Device Management)

Resources:-

Update iphone

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

Learn how to update your iPhone via recent Macs here:-->

Update your iPhone, iPad or iPod touch with the Finder – Apple Support (UK)

or older Macs and PC here:-->

Update your iPhone, iPad or iPod touch with iTunes – Apple Support (UK)

KB Article - Iphone will not update

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