the valves shut off the water flow so that you can disconnect the hoses and remove the canister from under your stand for easy cleaning. If you just want to shut off your pump (for water change purposes), best thing to do is to turn it off by slidding the switch which is located at the base of the filter. Do not turn off the valves while the pump is still running.
after you have removed the canister from the main drain lines. (by twisting off the tubes at the junction point) you can then lift that canister off of the base containing the pump and take it to the bathroom to remove the old carbon and clean the sleeve, etc.
From there, the easiest way to remove the top is to turn one of the valves to full open, this breaks the vacum and allows you to easy remove the top for cleaning.