Once you are signed in to the JustPark app, select 'My Bookings' and click on the booking which you wish to amend.
Then scroll down the page until you find the option 'Extend Parking', if you wish to prolong the time of your booking.
As shown below, input the dates and times you wish to extend your parking. This can be done after you have already parked.
After this step, please ensure that you make any extra payment required for the added time.