Hey Lara_22
If you have bought a normal train ticket, i.e. not an saver ticket, then you can also take an earlier or later connection.
If you bought a saver ticket, then you have to buy a new ticket and get the saver ticket refunded.
In principle, you can make an exchange at the SBB counter or by phone at the Railservice. Tel: 0848 44 66 88 CHF 0.08/Min
Have a nice day, SandrineS