How much novelty should I add to my previous paper to get it published in New journal?
This seems unlikely. I doubt that an editor of a reputable journal would accept it, knowing the details. And it would be an ethical lapse to obscure the details, of course.
You can, however, write an extension of the old work, citing it as usual. But the new results would have to stand on their own to pass review. If they don't, then an option is to continue work until you have something that editors and reviewers (not readers here) find sufficiently &quot;interesting&quot; and novel.