Once you make your FP+ selections, the park you made them for will show up on your My Reservations and Tickets, My Itinerary, FastPass+, and My Plans pages in the
MDE website or app. That doesn't mean you have to actually go to that park that day (or go to a park at all that day). You have a 14 day window to use your days (and side trips if adding the Water Parks and More option) from the first day you use it.
But, if you book four FP+ selections for days 1, 2, 3, 4, and 6 and then decide go to the park on day 5, my understanding is that your FP+ selections for day 6 will be dropped because you no longer have a valid ticket for that day. So, if your intent is to add another day to your ticket and keep your FP+ selections for day 6 you should do it
before you enter on day 5.