How to update my phone ?

First see if your iPhone is eligible for updates

Think about the iPhone 6, or the iPhone 5S.

The iPhone 5S/6/6plus cannot update further then iOS 12.5.7

The iPhone 5C cannot update further then iOS 10.3.4

The iPhone 4S cannot update further then iOS 9.3.6

The iPhone 4 cannot update further then iOS 7.1.2

The iPhone 6S/6Splus cannot update further then iOS 15.7.8

The iPhone 7/7plus cannot update further then iOS 15.7.8

The iPhone SE (2016, first gen) cannot update further then iOS 15.7.8

The iPhone 8 cannot update further then iOS 16.7.1

The iPhone X cannot update further then iOS 16.7.1

Which iPhones are eligible for upgrades?

For iOS 15, the iPhone 6s and above can run iOS 15 and all the version upgrades of iOS 15, until the newest version iOS 15.7.8

For iOS 16, the iPhone 8 and above can run iOS 16

For iOS 17 the iPhone XR/XS and above can run iOS 17

For the iPhone SE: the second and third gen can run iOS 17

If your iPhone is eligible, please go to settings>general>software update. Turn off low power mode and connect to a wifi network

If updating wirelessly is not working, please update via a pc or Mac instead

Update your iPhone using a Mac with macOS Catalina or higher

On a Mac with macOS Catalina or higher, you will use the finder in order to upgrade your iPhone. Make sure to Back up your device before starting the upgrade process.

On a Mac with macOS Catalina or later, you will follow the following steps:

1. Open a finder window
2. Connect your device to your Mac via a lighting to usb A or a lightning to usb c cable On the iPhone 15 series, use an usb c to usb c cable.
3. Select your device in the left sidebar of the finder window
4. click general, then click check for update
5. Click download and update.
6. If asked, enter your iPhone passcode

Update your iPhone via a Mac running macOS Mojave or earlier, or on a Windows PC

On macOS Mojave or earlier, or on a windows machine, we won't use the Finder to upgrade your iOS

Instead, we will use iTunes. Make sure to Back up your device before doing any update or upgrade to your phone.

1. Open iTunes
2. connect your device to the computer with an lightning to usb a or a lightning to usb c cable. On the iPhone 15 series, use an usb c to usb c cable.
3. Click device button in the iTunes toolbar
4. In the summary tab, click check for update
5. Click download and update, follow The on screen instructions
6. If asked, enter your iPhone passcode.

https://discussions.apple.com/docs/DOC-250005397