How to update iOS 15.0

If you have an iPhone 6/6PLUS/5S you cannot update further then iOS 12.5.7 due to hardware limitations

If you indeed have an iPhone 7., this one can update to iOS 15.8.2. iOS 15.0 is not signed anymore by Apple so you can directly update to 15.8.2

Go to settings>general>software update and make sure to turn off low power mode and to connect your device 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.

See this link for a user tip that I have written for further reference about updating your iPhone

Updating/upgrading your iPhone - Apple Community

Update my phone