Time and Tide wait for none.... I don't like this saying sometimes, especially during exam days, when time seems to go fast during break/leisure times and during studying for/writing the exam. Time is a factor which cannot be stopped. It goes on its own accord. If I had the power to change something, that would be time, at my wish. I can rewind to the past and see things for myself, for instance, my parents' marriage ( like it comes in Harry Potter and the Chamber of Secrets & Prisoner of Azkaban ).
I could go back in time and see events I would want to 'relive' again or 'watch' for the first time, for example, the 1983 world cup when India won (Note the difference between relive and watch). When I grow to be old, I could rewind my life back to my childhood days till it goes to the old age and repeat again and again like the 'for' loop in computer science, having power to change only minor events. It should be programmed in such a way such that it does not affect others. This might be a wild idea. But I could write my public examinations again by rewinding back to january, having just enough time to prepare for it, now that I know the question paper and get All India first (:P). If I can change time, I could go back and complete my half-learned cycling practices. Another logically true but fictitious thing, if I had the power to change time, is I would be knowing all the things (eg: calculus) when I go back to childhood, since my memory would not change. So I would not be going to tuitions, naturally conserving time for more free time, in which I could learn new things or simply enjoy my extra time in my life.
I could go back and experience those wonderful memories that I had at my school, with my friends. I could rewind then and there to avoid late comings. I could rewind the night before the exam repeatedly until I become thorough with the portions.
Sometimes, I like Voldemort from the Harry Potter series, not because he was evil, but he would have remained immortal if he had not been killed. Well, in this case what I have said, it is not exactly immortal but I am re-living my life again, with my beloved ones again.
Having power over time, can make one 'perfect' illegally. I wish I had the power to change time, but alas, no friggin-body can change it!