How do I update my iPad to iOS.15?

Use a computer or the Software Update function in Settings. If you're referring to a regular iPad 2 or any unit on 12.4.2 through 12.5.7, it's not capable of 13 or above.

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Go to Settings > General > Software Update. If your iPad is eligible for an update, you will be prompted to do so. You can find more information here:

Update your iPhone or iPad - Apple Support