How to update iPhone 6 Plus to iOS 12.5.5?

Your battery level shows less than 50%. It has to be at least that high, or plugged into a charging cable before you can apply the update.

That said, your image shows you do have the phone plugged in (lightning symbol). Then the question is, do you have enough free space?

Go to a public Wi-Fi hotspot or your home network, or use a computer's Internet connection.

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