Hi,
You could have an actual reason or a guilty reason, but to ensure that the HR can reschedule the interview, you must give a liable reason.
Some of the points - 
- Stay calm
 
- Contact HR at least before the interview than later after INterview time.
 
- Try to be honest (if possible)
 
- Be responsible
 
- talk respectfully with HR regarding the problem and choose a wise date.
 
Hope it helps.