Honest answer, you can’t un-expose it. But you can stop the bleeding. Move your nsec into a signer so no new app ever sees it again, then delete it from everywhere else. Odds are nobody grabbed it, the risk is what happens as the network grows.
And if your identity really matters to you, the clean move is starting a fresh key that’s never touched software at all. Way cheaper/less hassle to do now than in 5 years. The KEY does either, restore your existing nsec or generate a new one on the device.