What to do with old blog posts

I have been blogging for a few years. I get about 8K visitors a month. I have old posts that get zero clicks. I never did any SEO with them. Is it better to improve them and do SEO optimization, or delete them and re-purpose content for new posts? I'm not sure if Google would like the old posts to show many years of writing or if it would not like the improved old posts because they are old. My niche does not become inaccurate with time. It is not time-sensitve.