You can look into using the SlingPostServlet functionality, which is the content CRUD API, to modify the page. https://sling.apache.org/documentation/bundles/manipulating-content-the-slingpostservlet-servlets-po...
If you are planning to update the content on a frequent basis, you might consider making the content changes on the frontend, since the page cache may become a problem.
But for the post servlet, you just need a service worker with a token and authentication to make the modification. In general, I would caution against doing too much automated manipulation directly on a published page. You might consider posting the content into the JCR then allowing a workflow to update the content so it can version the content, make the update and notify someone the update was made and published.