Why my phone not updating

What happens when you try?

Tried an update using a Mac or Windows computer, as described in the following link?

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

There are many reasons:

1. iPhone is not connected to the Internet via Wi-Fi (not Personal Hotspot)
2. iPhone is not connected to a power supply (not using Low Power mode)
3. iPhone do not have enough free space (4-5 GB)
4. iPhone is running an iOS Beta version
5. .....

Best you tell us what you did.