This should be a simple job, remove the small inspection plate on top of the outer cover of the gearbox, you can then push the inner cable in a bit and remove it from the actuating lever, make a note of the routing through the frame and simply refit the new cable. You will probably have to slacken the adjusting nut and screw on the primary drive side of the clutch to allow the cable sufficient slack to go onto the handlebar lever.