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
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Once your book is set up, you go here to upload:

https://leanpub.com/YOUR_BOOK/upload
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...making sure to replace "YOUR_BOOK" with your book's unique slug.

Hopefully those resources will show you what to do!