Question

Hi, I have been reading the brief history of time by Stephen Hawking and have been greatly enjoying it. In the second chapter he mentions the fact that time is slower when in proximity with a large mass such as the earth. So a twin on top of a mountain would age faster than his counterpart at sea level. He credits this phenomenon to the energy of light, saying that the energy of light decreases as gravity decreases. I was wondering how the energy of light can vary but its speed remain constant. Since high school I have been taught that if you change an objects energy you are also changing its gravitational potential or velocity. Any reply will be greatly appreciated.
Abhi.