You will have to wait for the "Single Trip Booking" window to open before being able to make edits to individual trips pick-up/drop-off MRT stops.
During the "Single Trip Booking" window, you will be able edit your single trip details on a daily basis, up to 3 hours in advance of the ride time. Seats are subjected to availability on a first come first served basis.
* You will only be able to make a booking onto existing MRT pick-up/drop off points.
Click here to access "Single Ride Bookings" FAQs