I want to update my iPhone 8 plus from version 14.7.1 to iOS 15

Start here: https://support.apple.com/en-us/HT204204

sajamie Said:

"iOS 15: I want to update my iPhone 8plus from version 14.7.1 to iOS 15"

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Use a Computer and Ethernet:

Start with attempting an update through use of a computer. If you cannot use Wi-Fi, then use Ethernet.  See my User Tip: Update your iPhone Using a Computer - User Tip. On the computer, you would connect it through Ethernet, going directly into your router. For a Mac (or PC) that does not have Ethernet, this would be the adapter to use: Belkin USB-C to Gigabit Ethernet Adapter - Apple. If you get the same error, the model is likely just not compatible.