We'd be pretty sad to see you go, but we totally understand.
Click on your username in the top right corner, then click "Edit Studio". In the first tab, entitled "Account" you'll see a big red button labeled "Cancel Plan". Click on that and then confirm.
Your plan will be active until the end of the period, at which point it will revert to a free plan. Free plan users can only access their most recent album and store up to 50mb of images (including client uploaded images).
Just be warned that your clients will be unable to access your older albums too. Please don't cancel your plan if you still have clients actively proofing albums!