Everyone is saying updates which is fair, but you shouldn't only look at posts already getting traffic like
@ads studio mentioned. The real easy wins are the ones sitting on page 2 or 3 with decent impressions in GSC but barely any clicks... quick title tag tweak or stealing some subheadings from competitors usually pushes them to page 1. Also don't be afraid to prune the actual dead weight. If a post has zero impressions after 6 months, just delete it and 301 redirect it. Keeping useless bloat hurts overall site authority.