To upload a book and publish it on Leanpub, your book needs to be in our Upload writing mode. Here is a walkthrough from our Help Center:
http://help.leanpub.com/en/articles/2932466-getting-started-using-the-upload-writing-mode
And here's a video:
https://youtu.be/8sSi0PLwIDo
Once your book is set up, you go here to upload:
https://leanpub.com/YOUR_BOOK/upload
...making sure to replace "YOUR_BOOK" with your book's unique slug.
Hopefully those resources will show you what to do!