How do I update my iPhone?

First make sure if you have an eligible iPhone. If you have an iPhone 5S, 6 or 6 plus you cannot update further then iOS 12.5.7

If you have an iPhone 6S or 7. you cannot update further then iOS 15.8.2

If you have an eligible iPhone go to settings>general>software update and make sure that low power mode is turned off and that you're connected to a wifi net work

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.

See this user tip I have written for further reference about updating

Updating/upgrading your iPhone - Apple Community

Without knowing what iPhone you have, what the current iOS version is on your iPhone and what you are hoping to update to, it's difficult to offer much help.

You can here from an article about iOS 15. If for example, you own an iPhone 6, your phone will be on iOS 12 and it is NOT capable of being updated to iOS 15.

Go to Settings > General > About > Model and Version and report back.