I, want to leave USA and my visa extension which I applied for in march 2016 has not arrived yet what do i do

You are free to leave the United States. However, if you depart before a decision is made, you risk your application being denied due to abandonment. You may accrue unlawful presence is such instance if your authorized stay has expired. Contact USCIS before departing to inquire about the status of the request and to update your mailing address.