To set the price for a book, select your book in the Leanpub menu and go here:
https://leanpub.com/YOUR_BOOK/edit_pricing
...making sure to replace YOUR_BOOK with your book's unique web address.
Here is some information on Leanpub innovative "variable pricing" model:
You can find a link to the Pricing page for your book under "Store" on your book's Overview page: