You can try this link: https://www.mathworks.com/discovery/edge-detection.html
and this video: https://www.mathworks.com/videos/edge-detection-with-matlab-119353.html
Or since your image is purely black and white, you can try this:
img = imread('yourimage.png');
subplot(1,2,1);
imshow(img);
img = imcomplement(img);
subplot(1,2,2);
imshow(img)
