Is any body know which way I can change car pick up time? I tried calling so many times, only reply "try call again later "

You have to call and keep calling and leave messages. Look up fox car rentals actual phone number. You will be able to get ahold of someone there. If you are going through a third party website you will just get thrown around. And don't buy third party insurance. Only deal with main payments and stuff through fox

If it is only a few hours, I believed you are safe to head there a couple of hours earlier or later.

Try to go online and change itinerary to move time up. Otherwise you may be responsible for charges due at original time of pick up and/or vehicle may he released to other party. I've used contact numbers on webpage but yes they're usually busy.