The engine ECU controls the fuelling and ignition of the car based on a pre-programmed map and various sensors around the car (air temp, engine temp, exhaust sensors etc. etc). Higher RON fuel allows the ignition timing to be advanced which gives more power. Too much though and the engine will knock (pink) which will damage it. By using a knock sensor the ECU will retard the timing to the correct level.
If you put in higher RON fuel the ECU will not know this and by default will only try and advance the timing infrequently. By resetting the ECU all data is lost and therefore it has to relearn the correct settings. It starts off by using very conservative settings but will increase the settings at a faster rate than normal. So after a tank full of fast driving the ECU should be giving you the maximum performance.
You reset the ECU by removing its power. This can be done via its fuse, if it has one, but the definite way is to disconnect the battery. It should only need to be done for a minute but as it is difficult to tell if the ECU has been reset it is recommended that you leave it off for 5 minutes to be sure. Also you need to discharge any power being held within the electronic circuits, which may still provide the ECU which enough power to keep its memory, by trying to draw some power by turning on the lights, using the horn or pressing the brake pedal.