How do I update my iPhone 8 Plus?

What exactly happens when you attempt to update your iPhone 8 Plus? Do you get any error notifications?

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Make sure that your iPhone is setup to meet the following requirements before you attempt to update the iOS version over a Wi-Fi Network. Open Settings ➡️ General ➡️ Software Update: If there is an update available for your iPhone then it will be listed here. Follow the on-screen instructions to download and update your iPhone. 

• Your iPhone must have a fast Wi-Fi connection or a 5G cellular connection. You cannot update over a cellular hotspot
• The iPhone can not be in Low Power mode
• The battery state of charge must be above 50%
• Your iPhone must have at least 12GB or more free storage space
• Your iPhone cannot be connected through VPN
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).

Axel F.

As your iPhone is capable of running iOS 16, the iOS 15 option is not available to you:

You can easily update to 16:

Update your iPhone or iPad - Apple Support