You can edit purchase links in Books > select the book whose links you'd like to edit > click the Purchase Options tab.
If you'd like to get a new regional Amazon link, you should be able to locate the Amazon website in your browser and change the region you're located in directly on the Amazon site. Search for your book and copy to URL at the top of the Amazon bookpage.
Return to the Tertulia editor (Purchase Options), click the bar that says Amazon, paste the updated link into the URL field, and click Save Changes.
Learn more about setting up your books on your website here!