In a conference paper, in order to prove the $\mathsf{NP}$-completeness of a problem, I wrote the stupid sentence "It is clear that the problem is in $\mathsf{NP}$. So we will prove that it is $\mathsf{NP}$-hard". In fact, it was not clear at all. It even seems to be an open problem. For the targeted audience, it's not a big deal because the main result is the $\mathsf{NP}$-hardness, and thus the impossibility "difficulty" to design a polynomial algorithm.
But, of course, I want to correct the mistake in the journal version.
My question is: In a journal paper, how to point a mistake in the conference version and correct it?
Should I write: "There is a mistake in the conference version: ..."? Or should I state the correct result without saying that the conference version was wrong? Any advise is welcome.