Answer:

Hello!
To open the eBooks you have downloaded, you need a proper eBook reader.
If you are using a mobile phone, you can download the Kindle app from the Play Store.
Alternatively, you can also use other apps like Adobe Digital Editions or any good PDF/ePub reader, depending on the eBook file format.
Make sure the app you choose supports the file type (like PDF, ePub, etc.) of your eBook.

Hope this helps!