Once you've located the correct sermons page, click on the page title to edit the page itself.
The Year page contains an overview of the sermon series, using the relevant title image for each one, with some text to explain with a sub title and a monthly timeframe. Each of these Title images link off to their respective blog pages, containing all the sermons within that one series.
Sermons is a "landing page" for all the sermons (in the top menu and top section of pages). It's like a road sign, directing people where to go.
Sermons-2020 contains all sermons for 2020.
Sermons 2018 and Sermons-2019 the same, respectively. These should not need any further editing.