To bend the pitch down, drop the corners of your embochure (let the pitch go flat). Try not to simply cover the hole; that's the fast, easier way, but the tone suffers. The Trevor Wye Practice Book, Tone, has a couple pages with exercises related to this. There are some short exercises in which alternate notes are to be reached not by using that note's correct fingering, but by bending. In music which does not require a contemporary, 'effect' use of this technique, it's useful for assisting intonation, like in an ensemble setting.