Let me preface this by disclosing that I am not a site administrator, so please don't take what I say as gospel. When you click on a post, towards the bottom, right above the "Comments" section you will see a row of icons. On the left side you'll find "PM," "Find," and "Add Rep" and on the right side you'll see "Like," the green "Working" button, the red "Not working" button, and finally "Report." Click on "Report" and under "Report reason:" choose "Broken Link" and then "Submit." You should see a popup saying it's been successfully reported to a moderator.
I'm not sure exactly how this works on the back end though -- I'm assuming it all goes to an inbox of broken link reports and if the mods have a backup they (eventually) repost it using the repost bot. Additionally, you can also use the red "Not Working" button to add a tally to the ratio of Working:Not Working. I don't think this has the same effect as reporting it, but is mainly used to let other people know if the link might still be working vs not working. Currently using the report feature is the best way I know of reporting broken links on the site. Hope this helps!