It is simple, I went through this myself. You can pick two categories. Here, I will just copy and paste the email Kindle sent me so you can see what you have to do.
We can add a total of two categories that aren't currently available to select in your KDP Bookshelf, as long as they already exist in the Kindle Store. If you'd like us to manually change your category, you'll first need to remove the current categories. Here's how:
1. Visit
https://kdp.amazon.com and log in.
2. Check the box at the left side of the book and then click on the "Actions" button at the top left, then click "Edit book details."
3. Scroll down to the "Target Your Book to Customers" section and click "Add categories."
4. Choose the categories on the right side of the screen, then click "Remove category."
5. Scroll down the category list on the left side of the screen, and choose the last category - "NON-CLASSIFIABLE."
6. Click on "Save."
7. Scroll down the page and click "Save and Continue."
8. You'll then be directed to the "Rights & Pricing" section. Check the box at the bottom of the screen to accept the Terms & Conditions, then click "Save and Publish."
After you have done this, reply to us with the exact path of the Kindle Store categories you wish to add as shown in the example below, and we will add the categories for your title.
Example: Kindle Store > Kindle Books > Religion & Spirituality > New Age > Mysticism
If you have other questions about Categories, check out our Help page:
https://kdp.amazon.com/self-publishing/help?topicId=200652170
I hope this helps. Thanks for using Amazon KDP.
[email protected] (I think that is the correct email address for them, if not, you can email them through the KDP page)
The only downfall is that if you already have categories set up, this will only take effect on Amazon.com. It will not take place in other countries. So best to get it right the first time if you can.