This is not a problem; however, it must be done at least 12h before the scheduled time. That could fluctuate depending on the agency policy. If you don't manage to officially change your drop- off time, and you are late you are going to be charged a late fee. All the agencies are open from 06:00 to 23:00 seven days a week. If you do need to make a booking or change your collection time or cancel your car outside of those operating hours, online communication is effective. Some of the agents do have a 24h customer service line, however, do make sure you add your flight details to your initial booking, that way the agent will be able to track your flight and adjust accordingly if there are any delays without you having to contact them. Search car hire on KAYAK