Downloading the ebooks is easy. Select the right versions (probably epub), press the buttons and accept the download (if asked). Now you have the books.
However, you won't be able to open and read the files unless you have an e-reader app installed. There are many ebook reader apps available for free. Just go to the app store of your device (Google Play etc.), search for an e-reader and download it. After that you can start reading.
If you have an iPhone or iPad, you have the e-reader preinstalled. After downloading the ebooks, start the Safari browser, search for downloaded files and click the ebook to open your Books app.
Should you need more assistance, it is probably best to ask someone to show you how to do it.
Bless you! 🙂