Software update

You might be unable to update your iPhone for one of these reasons. 

• Not enough available storage space on your device 
• Downloading the update takes a long time 
• Unable to reach the update server 
• The update doesn't complete 

Hope these Apple Support links can solve your issue. 

• If you can't update or restore your iPhone, iPad, or iPod touch 
• Get help with over-the-air iOS updates