Get the free External Image Editor for Photos from the App Store and install. then you can set the editor of your choice as your default editor.
Warning: if, in your editing, you change the extersion of the original image, i.e. like from jpeg to .psd (Adobe Photoshop Elements) you must save the edited photo to the Desktop and import from there. If you just save the library will not recognize the edited version. So if I edit in PSE and it results in a .psd extension I flatten the image and save it as a jpeg (if the original was a jpeg).
You will be able to choose from any image editor that's in your Applications folder: