​@Chi Chung LO 

You can change reservations only before departure, in this case before 10:10. In Italy, you can also install the Trenitalia app on your smartphone, load your ticket and change the reservations in this app. After this, no change or refund is possible on Eural/Interrail website.

may be I can join this train 09:13 Malpensa Aeroporto T1 to 10:07  Milano Centrale (landing is 07:40 by airline info.), I know the best should be 09:37 arrive, but not sure how long about immigration and some else

I just worry arrival at 10:07, just only have 3min to change. For the first time beginer is enought or not?

Is it nearby both platform Malpensa Express and Milano Centrale?

​@Chi Chung LO 

It is not possible to change in 3 minutes in Milano Centrale. You should reserve in the 11:10 train. If you arrive at 9:37 in Milano Centrale, you could exchange the reservations for the 10:10 train if seats are available. Trains could also be sold out, especially on Mondays morning.

​@Chi Chung LO 

It is not your question, but, if you want to visit some museums in Florence, it is also advisable to book time slots tickets in advance.

Interesting, does this really also work for pass reservations? I'll have to remember that.

​@rvdborgt 

Yes, it works, but only with a Trenitalia ticket, for example bought on Raileurope. It doesn’t work with an ÖBB reservation. It is better to create an account on Trenitalia and log in in the app.

You must indicate the PNR and your e-mail address or the CP code. You obtain the new PDF ticket with a mail directly from Trenitalia, the QR code is also available in the app. Such changes are not communicated to the original vendor and, as indicated earlier, no exchanges or refunds are possible later on Eurail/Interrail or Raileurope websites. 

Thanks ​@Danhiel, very useful.