We aren’t scientists, but the quick explanation of why leaves change is that during the spring and summer, the leaves are rich in chlorophyll, which absorbs sunlight and turns it into food for the tree through photosynthesis. This gives the leaves the bright green color you see during this part of the year.
During the fall, as the days become shorter and cooler, the leaves stop this process and the chlorophyll breaks down, which causes the colors of the leaves to change to the red, orange, and yellows you see in the fall.