Keeping your website fresh and active is about making changes to or adding new content to an existing page is an often overlooked part of managing a website. It should be part of your routine as the site owner to look through your site every couple of weeks to see if anything new can be added.
If changes are made, be careful not to change the primary topic of a page. The page might have been optimized for search engines based on its particular topic. Adding too much content or changing the topic can change the focus in the eyes of the search engine and probably the visitors, too.
If the content is changed sufficiently, you would need to re-optimize the page, which can take some time and effort. Always see if you can add the content with different wording or maybe creating a new page altogether.