If you return to your vehicle earlier, and if this possibility is allowed, depending on the city, you just have to stop your session from the application and you will be charged only for the effective time, according to the following process: (i) Open your application and (ii) click on the current session, (iii) Then click on "Stop my session".
If you stay longer, you have the possibility to extend the duration of the Parking Session before it has ended, always within the maximum allowed parking time, according to the following process: (i) Open your current session, (ii) click on "Extend my session", (iii) select the new end time and (iv) confirm. Your session is automatically extended by the additional time.