Pro tip: Click here to process a return with Amazon. Or, contact Customer Service for more assistance. You're refunded the money you spent on your Kindle Book.
Returning a Kindle eBook gets done in the same way you return a physical parcel. The difference is that the process is much quicker and there's no shipping required.
Kindle eBooks are available for both Kindle devices (found on Amazon, here) and non-Kindle Devices. You've got to look at your 'Digital Order's tab under 'Your Orders' to find the book you want to return.