It’s in your owners manual, but typically on Lexus and Toyota you turn “ON” the ignition, press the odometer button until your total mileage is showing (not your trip A or trip B). Turn off the ignition completely, while pressing in and HOLDING the same button as before (the odometer button), turn the ignition switch to “ON”. In about 10 seconds you will see the maintenance light flash fast a number of times, after about 20 seconds you can let go of the switch and start the car. It should be reset for another 5000 miles.
Always change your oil at least at 5000 miles. If you are having your oil changed at a "quick lube" type place, place close attention that they are REALLY changing your filter as the RX is somewhat of a pain to change and many low cost places “leave it for the next guy”. Also make sure BEFORE YOU LEAVE you are not losing oil under your car (stripped oil plug).