How could you decrease hdl?

You don't want to. Hdl stands for high density lipoprotein cholesterol - the good ('h' from 'healthy') cholesterol that 'cleans' the endothelium (blood vessel lining) of fatty particles that lead to blockages. In fact, one would want to have the highest HDL possible, at least 40 mg/dl for men and 50 mg/dl for women. Being lean and exercising, not smoking are the bat ways to increase your hdl.
You don't. Doctor's typically try to lower the LDL also called the "bad cholesterol". Hdl is often referred to as the "good cholesterol". Doctor's can use medications to raise the hdl. There is no benefit to lowering the hdl.