H81M what? Gigabyte, ASUS? There will be a lever, or a tab that you need to either flip or push over. Your video card is hiding it. As GPU have gotten larger these tabs / levers have become more and more of a problem, and become harder and harder to get to. Google your exact motherboard, and and you will be able to see what you are dealing with. There will be a retainer there of some sort.