I booked a flight with Frontier, then reserved a rental car with enterprise. Since then Frontier has changed my flight times significantly. Now I need to change my rental pickup & drop off times. How do I talk to a live person with Enterprise?

You can go online (enterprise.com) and modify your pickup or drop off, date and time however it may result in changes to your rate, taxes, etc.

You can call the branch you booked the car at! Don't call the 1800 number

Usually the airport locations allow a grace period so you can pick up the car anytime that day. Then the actual time you pickup/checkout, you then have to turn the vehicle back in at the same time on the return day (usually with 30-minute grace period).