You can cancel your first travel day any time before the start of that day (00:00 CET). Go to your Pass in My Pass and tap 'Cancel travel day'. You can then choose a new first travel day by adding a journey to your Pass on that day or adding a new travel day in My Trip.
You can also choose an earlier date when using a new travel day, which will then become your first travel day, provided you still have travel days available on your Pass. Your Pass validity period will change to match this new start date.
Please note that some European countries do not observe Central European Time (CET). This means there can be a difference in the start and end time of a travel day when compared to your local time while travelling.