Kennedy did just that, sort of, going on Chris Cuomo's CNN show and explaining that he'd misheard the question and wants everyone to know that, yes, it was Russia who hacked the DNC computers.
Now, mind you, this got him a lot of praise -- a Republican actually acknowledging he was wrong. And yes, that was swell. The problem is...immediately after he kept insisting that there was evidence Ukraine may have tried to manipulate the 2016 election -- this despite, a) his sources for this were two magazine articles which Cuomo pointed out only asked questions, and weren't giving "evidence," and b) Kennedy himself had been directly briefed by U.S. intel services that the idea of Ukraine attacked the U.S. was Russian disinformation -- and yet he's still trying to raise that as an issue.
And one more thing. His initial claim that maybe Russia didn't attack us and it was Ukraine, that was said on "Fox News" where all the right-wing conservatives heard him. His "correction" was on CNN, which so many right-wing conservatives deride and avoid like the Plague. So, it's pretty likely that most of the people who heard his original lie did NOT hear his slight "walk-back."
The other John Kennedy who served as President of the United States won a Pultizer Prize for the book, Profiles in Courage. This John Kennedy showed what the lack of that looks like. Even in his supposed "correction."