Google is indexing pages that I used like a year ago and now they are 404. I want to add a noindex tag to them, but can't figure out how to open the code for a desired page so I can put in the tag. In addition, should I do anything with those 404 pages so I have a better seo ?
If they are 404, you can't put a noindex on them! You can put
rel=nofollow on the links that link to the removed pages.
It also helps to have a sitemap so that Google can track changes more easily.