One time in college, I scored an interview for an awesome entry-level job to kick off my post-college career. But I had to cancel due to a scheduling conflict with a summer class. Bad idea. I couldn't reschedule that day, and I never heard from them again—except when I called to see if the position was still open (it wasn't).
Moral of the story? Don't reschedule your job interviews! You could end up like I did, without a chance in hell at getting the job you so desired.
But what's that you say? What if you have a legitimate emergency and have no other choice but to reschedule? Okay, go ahead if you must. Just make sure you set it up as close to the original date as possible. You want them to know you still really want the job, and you don't want to forfeit it to some other candidate because it took you another week to get back in for an interview.
If you absolutely have to reschedule, be as flexible as possible and accommodate the employer's schedule. These days, you probably need the job more than the employer needs you, so if they offer an alternate date and time, take it—even if you have to cancel three other appointments and borrow your brother-in-law's car to get there!
Is it OK to Walk Out of a Bad Interview?
How to Get Time Off for a Job Interview
Dumbass Mistakes New Grads Make in Job Interviews
Ready To Jump Start Your Job Search?