Update iOS 12.5 on my iPhone

An iPhone 5 isn't capable of 11 or above.

An unit running any version of 12.5 isn't capable of 13 or above.

To update it to the latest compatible release, use a computer or the Software Update function in Settings.

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The iPhone 5 can be updated to iOS 10.3.4, no further.

As explained above the iPhone 5 is not compatible with iOS 11 or higher.