This is an ambiguous question. What is the initial range of your image? Do you want to change that? Or just change how it's displayed? One way is
imshow(grayImage, [0, 127]);
imshow(uint8(127 * mat2gray(grayImage)));
The last line of code will scale the image range [min, max] to [0,1]. Then it multiplies by 127 and converts to uint8 so it will display properly in the [0,255] range. I believe Shoibur's code will display as all white because newImage will be a double and imshow expects doubles to be in the range 0-1 and anything above 1 will get clipped. So that's why I converted to uint8, which uses a [0,255] display range by default.