I need to update my iPhone 6 Plus to iOS 14

If you are asking about the iPhone 6 Plus in your equipment profile, it cannot be upgraded past iOS 12. It is too old and lacks the required internal hardware. No fixes other that getting a new phone.

If asking about a newr phone: Update your iPhone or iPad - Apple Support

An iPhone 5s or iPhone 6 isn't capable of 13 or above; they require at least 2GB of RAM.

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