Is the ticket you bought a normal ticket? In that case it is valid a whole day and you can just take an earlier train. You are in that case not limited to the train you selected during booking.
If it is a supersaver ticket then I am afraid that a refund or change is not possible. You will have to buy a new ticket.
So have a look at the type of ticket first.
Note that if you booked the other part on Trenitalia that you could have booked Spiez - Florence there. Splitting the ticket in Domodossola is usually only done if you have a Swiss Travel Pass.