A quick check at Korg's websites doesn't show any place to download instructions (for anything). The picture of the AT-2 (from their archives) shows a rather large button marked "Calibrate" which likely does "something." Pushing the button may let you discern how it works.
A SWAG would be that pressing the calibrate button will light up an LED for one of the "A-frequencies" along the top of the display. Repeated pushes should advance to the next setting. You want the light at 440, of course, the last time you punch the button.
Once the correct "A" is selected, the AT-2 probably senses the "closest tone" and the same LEDs now indicate what scale note you're close to.
But I'm just guessing.
If you're convinced it's "wrong" you're unlikely to make things worse by experimenting.
Several places still advertise the AT-2 for sale, even though it appears to be on Korg's "obsolete" list. If you can find a local seller you might be able to "examine" one and sneak a look at the destruction manual(?).