a know it all
A know-it-all is someone who can’t wait to jump in and say you’re wrong, even if they’re the one who’s actually wrong. They’re always trying to fix your mistakes with their own dumb ideas.
'You spelled it wrong! I know how to spell it!' says the person who spelled it wrong themselves.
'I didn’t say that! I said the opposite!'
'You thought it was a cat? It was a dog, and I know because I saw it!'