How do you update iPad to iPadOS 12?

That model is an iPad 4 and not capable of 11 or above; it's also not subject to the issue 10.3.4 patches, so the latest version for it is 10.3.3.

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If you're referring to an iPad 4 or earlier, or a first generation iPad mini, you can't. Otherwise, use a computer or the Software Update function in Settings to update it to 12.5.4 or 14.6.

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