In the browser tools, say in debugging (any browser will do, but IE9 preferred) how can I access things like html5 canvas and modify individual pixels by typing commands from prompt ?

I know, it is possible to accomplish in miriad normal ways with preparing the markup and loading the page, but what is the shortest path ? The browser is running with some content, then I hit F12->Console-> what exactly should I type to cause a canvas to change ? Thank you