When configuring your trip, click on "Enable changes" at the top right of the "Customize" page. This will enable the advanced editing features.
If you wish to add a new stop, click on "+ Stop" which will be displayed between one stop and the next; you can select a location from the "Recommended stops", or search for a stop using the free search field.
If, on the other hand, you wish to delete a stop, click on "Manage" under the number of nights and on "Delete".
After each change, all the travel services affected by your change will be updated automatically. For example, if you add a new stop lasting two nights, the configurator will update the date of any return flight, extend any car rental, postpone hotel bookings for the stops following the one you added by two nights.
Before proceeding with the purchase, therefore, check how the other services have changed with particular reference to flights and connections between the various stops.