Apr 03, 2016 - 12:02 PM
Your ticket from Venezia Mestre > Venezia S. L. on 4/14 is a regional train ticket.
Per the rules listed on your confirmation email, Regional train tickets can be used for another date and/or time as long as it is within 2 months of the purchase date. You just must stay on the same route, and can only use it on another regional train.
You would change the ticket when you print it at the kiosk at the train station.