We're sorry you're having trouble figuring out where to write or edit your new Leanpub book.
The issue here may be that you have chosen our Upload writing mode, which is intended specifically for authors who do not want to edit their book on Leanpub. This writing mode is intended for people who have their own process for writing their book.
We recommend you switch to the "In my browser with a Markua plain text editor" writing mode" here:
...making sure to replace YOUR_BOOK with your book's unique web address.
Then, you can go through the walkthrough online here, where you can learn in a few minutes how to write in Leanpub: