When you're using the laptop, are all the vents unobstructed. Most laptops pull air in from the bottom and blow the hot air out the back. Are you using your laptop on your bed or pillow or something that will stop air from being drawn in the bottom. If the answer is yes, the the fix is easy, stop using it on your bed.
If the answer is no, it gets more complicated. It is likely your heatsink is full of dust. The best way to clean it is to disassemble it far enough to expose the heatsink. Then used compressed air to blow the dust out of it. When doing this, you'll want to keep the fan from spinning. If you let the blast of air spin the fan, it may get it going so fast that it damages the fan bearing.
The less effective way if...