What if opponent accidentally reveal opponent top deck

It depends on who saw the card, but the fix is to shuffle the accidentally revealed card back into its deck.

The top card of a deck is secret information — no one is entitled to it (CR 128). If a player accidentally flips over the top of a deck, that's a Looking at Secret Information [Warning] (TR 702.10). The remedy: shuffle the now-known card back into the randomized portion of its deck to remove the information advantage. If that's not possible, the card is revealed to all players and play continues.

If the erroring player accidentally reveals their own top card, the opponent who merely saw it gets no penalty — players aren't penalized for errors their opponents make (TR 702.10.a). The warning applies to the player who caused the reveal.