When you add a new sermon to your site, you can specify a single or multiple topics for the that sermon. You can do the same thing for an existing sermon by clicking on "Sermons" in the left side menu, highlighting a sermon in the list, then click on "Edit" under the sermon title.
When you are adding/editing a sermon, there will be a box on the right-hand side of the "Edit Sermon" page that is titled "Sermon Topics". In that box will be an empty box with an "Add" button to the right of it. Under that box there is a link titled "Choose from most used sermon topics". If you click on that link, you will see all the topics that you have previously added to your list of topics. If you click on one of those topics, it will be added as a topic for the sermon you are adding/editing. You may select several topics for any given sermon. After you have finished your editing, you need to click on the "Publish" or "Update" button on the right-hand side of the "Edit Sermon" page.
If you need to add a new topic to your list of topics, you can type in a word or phrase for your new topic in the empty box with the "Add" button next to it, then click the "Add" button. The new topic will now show up under the empty box along with a grey circle with an "x" in it.
If you want to remove a topic from a sermon, click on the grey circle with the "x" in it (it will turn red when you hover your mouse cursor over it), and it will be removed from the list of topics for the sermon you are adding/editing.
You can use the same technique to add/select preachers in the "Preachers" box, or add/select a Bible book in the "Book of the Bible" box, or add/select a sermon series title in the "Sermon Series" box.