check this out

http://www.uscis.gov/graphics/services/factsheet/howdoi/f_english.qxd.pdf

On page 2, it talks about if you abosolutely can't make it to the appointment they scheduled. Since you just want to reschedule it to get some time for you, why simply call them? Of course it's a different story if you live very close to a local officeI. I don't. That's why I call them a lot.