Standard tickets are for a route, not a train. So what you have is a ticket Chur to st. Moritz valid on April 6th. You can use that for one trip from Chur to St. Moritz, and you are free to use any train on the route for that purpose.
There is nothing extra you need to do. Just find your train, get on and find a seat. There are no seat reservations on those trains.