Laptops usually don't allow overclocking, there are ways to do it anyway though.
One of the greatest risks by overclocking your GPU is that your whole laptop will die. The heatsink that is on the GPU can just about handle your GPU's current TDP, TDP is the heat your GPU is putting out. By overclocking the GPU then the TDP will increase, which might smoke the whole laptop as the heatsink can't handle it. You simply shouldn't overclock laptops in most cases.
Other consequences is lower battery life, which already have been mentioned.