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:
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!