Welcome to the forum. You are referring to the "oil life monitor". This forum is aimed primarily at the North American market Fiesta. There are several different procedures to reset the monitor depending upon the year and model of Fiesta. If you have the owners manual for your particular Fiesta, you can go to the index and look for the oil life monitor reset page. That is the best possible way to get the procedure. Or, just visit the nearest Ford service department and they can show you how to do that.