Resizing an image for KDP Book Cover

As previously mentioned, try reaching out to the artist to update the design to match the new cover dimensions. Otherwise, it's challenging to provide advice on your own since we have no idea how complex the design is. From what I can gather, it's only the width that needs to be updated, this may be easy or very difficult, depending on the design. In some cases, you can utilize automated options in the latest versions of Photoshop: select the empty area and ensure that a small portion of the selection touches the design, then use the Generate feature.

The original artist should have the files and be able to resize the cover. You may or may not be able to do so, depending on how the file was created. Increasing the canvas size just adds padding on the edges.

Photoshop (mostly) uses pixels to store visual information. Pixels are tiny rectangles (usually square) of varying color and tone. To enlarge an image, you must either add new pixels or use Photoshop's enlarging functions where the application interpolates new pixels that should match the older, smaller image. That works well sometimes and poorly other times.

A book cover would more often be created in Illustrator or InDesign, where scaling for a new layout is often easier and higher-quality.