Hi Kat00
As this train is a Regional Express train, you cannot reserve seats. Regional Express trains are mass transit, such as suburban trains too, and you can simply board a train that suits best your schedule and which departs from Como / arrives at Locarno within the ticket validity.
General speaking, seat reservations on Swiss trains are optional and only possible on IC trains. For border-crossing EC trains as well as PE (panoramic) trains, seat reservations are mandatory. The Swiss public transportation offers hundreds of connections every day. In the unlikely event you miss a connection, you simply take the next service, which usually departs in 30 or 60 minutes, sometimes even earlier. Use www.sbb.ch/en or the SBB Mobile app for updated train schedules and train information.