Question marked as
Try first to optimize the range of the mobile network, see -> If you see No Service, Searching, or SOS on your iPhone or iPad -> If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support
Then try to do the update again.
You need about 6 GB device storage and good Service & Wi-Fi.
If they are On, turn Off Hotspot & VPN.
Charge the battery > 70%.
See the following Apple articles:
Update your iPhone, iPad, or iPod touch
Update your iPhone, iPad, or iPod touch - Apple Support (IN)
If your iPhone, iPad or iPod touch won’t update
If your iPhone, iPad, or iPod touch won't update - Apple Support
You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC
Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)
If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder
Update your iPhone, iPad, or iPod touch with the Finder - Apple Support