さげる means you lower the degree of something. For example, スイッチを入れて温度をさげる means Turning on the switch to lower the temperature. 頭をさげる means you lower the height of your head. (or just bending down). くだる means you move in order to lower your height. For example, 山をくだる means going down a mountain. おりる is almost the same as くだる。 But おりる also has several other meanings such as getting off a vehicle, etc.