On the video detail page (the page that shows the poster, description etc), you can find the Download button here – it looks like a downward arrow ⬇️. Using your remote, make sure that icon is selected, and then click it. The app will then ask you to select the quality of the video to download. Choose the quality you want – the higher the quality, the larger the file and the more storage space/time to download. Once you choose the quality you desire, the download will start.