There are to ways to upload a book.
The first one:
1. Open a file manager.
2. Find the book you'd like to upload (you can upload books in FB2 or EPUB formats).
3. Tap on it and open it with Bookmate.
4. Tap on "Publish" and wait till the book is added to your library.
The second one:
1. Open My Books section in the app, scroll down a little bit and tap "Upload".
2. Select the book you'd like to upload and tap "+".
3. Wait till the book is added to your library.