I am unable to update my iPhone 8 Plus

iOS 14 is no longer signed and rolled out by apple. So you'll need to install a newer iOS version. If you indeed have the iPhone 8+ this iPhone can update to iOS 16. But, if you're still on a very old iOS version you must update via a pc or Mac. Beware that if you're having an iPhone 5C, this cannot update further then iOS 10.3.4. For the iPhone 6/6PLUS it is end of the line at iOS 12.5.7. This is all due to hardware limitations

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 about updating for further reference

Updating/upgrading your iPhone - Apple Community