Hi Damiano! 

You can cancel a travel day at any time before the start of that day (that is, until 23.59 of the night before the selected date). Select the Pass in My Pass and tap “cancel travel day”. You can set a new travel day by adding a journey on the desired date to your Pass or using a new travel day via My Trip.
If you use a travel day before your planned start date, that will be set as your first travel day and the overall validity period on your Pass will be adjusted accordingly.
 

Have a lovely day!

And again: it is VERY unwise to set the dates so long advance-do that as late as possible.