Change seat reservations from Napoli to Firenze for one day later possible?

Hello,

today I just ordered on the Interrail site a train Frecciarossa 9422 to Firenze and I just remarked that it is one day too early. I already called Trenitalia but they said only Interrail can change it. How can I do that? I don’t want refund, I just want to change it to one day later so I can continue planning the rest of the journey. 

I don’t understand why it should not be changeable on the same day of purchase and 2 months before the start. This thing is so complicated.

Please help me. I can’t plan without knowing. The messaging system via messenger is also a horror.

thanks in advance for your help.