Correcting a conference paper error in the journal version 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.
I think it is very helpful to point out if and where previous results are erroneous. I've done this myself (more times than I would have liked to). My style is to state the correct result, and in immediate proximity (above, below, footnote) a remark to the effect that "In the conference version [citation], Theorem X incorrectly stated that...".