Look for a reset button if there is one, or try messing around with the buttons. If that doesn't work, find our how your alarm clock works.
Does the alarm clock run on batteries? If so, did you try replacing the batteries. If that doesn't work, try to the battery run out completely and then replace it with a new battery to reset the time.
If the alarm clock plugs into an outlet, removing it from the outlet usually works to reset the time unless there is an internal battery. If removing it from the power outlet and plugging it back in does not reset the time there is an internal battery inside the alarm clock. Open up your alarm clock and look for an internal battery and replace it.