PROBLEMS WITH THE TIMER ON COMPUTER!

Hello, I have a brand new Acer, I had brought my computer to another city and put a timer on it so that the "Children" would not put any "Smut" or anything else on it that I didnt want on it. MY question is this..."How do I change the time for when the computer is to shut down or not? I had turned the time on when I was around the "Kids" but I "Obviously" forgot how to turn it bk off.