How you upload your ebook file to Leanpub from your device depends on which device you're using, the version of the device's operating system that you're on, and on the storage app you keep your ebook file in.
So, while we can't provide detailed support for every possible process out there, here's a simple example of what currently happens by default on iPad.
Once you've got your book set up in our Upload writing mode (here's the walkthrough http://help.leanpub.com/en/articles/2932466-getting-started-using-the-upload-writing-mode), you will navigate to this page to upload your book:
...making sure to replace YOUR_BOOK with your book's unique web address.
There, you will see something like this:
Click on the box that says "Drag your file here (or click to browse)". Then a box should show up providing you with some options. In this example, it opened on "Recents":
If you don't see your file here, try clicking "Browse" at the bottom right, which took us to iCloud Drive by default:
If you still don't see your file, try clicking on "Locations" at the top left, which took us here:
If you still don't see the app you're looking for, you can try clicking on "More Locations":
We hope that helps! If you would like to provide a tutorial for another device and have it published in our Help Center, we'd love to hear from you! Please email us with your proposal at firstname.lastname@example.org.