Software update not installed issue

What happens when you try?

Can you post a screenshot of the whole screen, it could reveal why you aren't able to update.

Make sure you:

• Are using a WiFi connection (not a hotspot).

• You have an active cell signal (You need both WiFi and cellular service to perform the upgrade).

• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.

• Are not using a VPN connection.

• Do not have power save mode turned on.

• Do not have a beta version installed.

• You have at least 6 gigs of storage available.

You can also check out this article for help -

If your iPhone, iPad or iPod touch won’t update

If your iPhone, iPad or iPod touch won’t update – Apple Support (UK)

Check 

• you are on a good Wi-Fi, 
• on charge with more than 50% charged, 
• no VPN installed 
• low power mode turned off. 
• Fully working cellular network connection via your SIM.
• Check Settings, General to make sure you have no Beta profile installed.

Resources:-

Update phone

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

Phone will not update

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