There is usually a number to call to request changes. Sometimes they want you to come in the first time to explain, but if you are out of the area, etc, you usually can send that in before hand and be excused for those days. Sometimes the days are tacked on to the end of your "duty week", sometimes they are just dropped.
Yes your employer has to let you off work. There are various compensation packages though. In other words, they have to let you off work, but they don't HAVE to pay you. Some employers do, some don't.
Also, just because you got papers doesn't mean you will have to report that day. You usually have to call the night before to see if you are required the next day.
Hope this helps. I have never actually served, but have had to go to the courthouse a few times on a jury selection.