If you’re using Apple devices, then all you need to do is download the zip file, extract it and then save it to your Apple device. Open it in iBooks and you’re ready to go!
For Android devices, you need to first download a free ePub3 reader such as Adobe Digital Editions and Infinity Reader depending on what will work for your particular android device. Of course, you can also choose to purchase an ePub3 reader which will probably give you a more seamless experience when using the eBook. But even the free ePub3 readers work well enough! After getting ePub3 reader, all you need to do is download the zip file, extract it and then save it to your Android device. Open it in your ePub reader of choice and you’re ready to go!