Comment on Sketchbook Pro 6 Tutorial by DraconianRain

you can't except either start over on it's own layer or erase carefully with the hard/firm eraser, erase the sketch around it if you made the mistake. But there is no short cut fix to drawing on the sketch layer. There's not even anything you could do in photoshop to rescue it properly. But you could try. The only issue with trying to lift lineart in photoshop from a sketch is it changes the linework to much and often leaves artifacts.