Change the month of your trip while it's already on

Hi, 

I won a France-Germany interrail pass. When I first login to the app I’ve been asked to choose a month for my trip but it also said that i could change it after all. 
I picked middle of august until middle of september but it appeared that i cannot find time to travel at this time. That’s why I want to change the date of my trip but now that it’s on I’m not allowed to. 
I’m afread I will loose my pass without being able to use  it. 
Does someone know if I can do something for it ?