Go HERE
Scroll down to [Entry #10] Removing Window Molding- Front Doors
Read, then (click photos to enlarge)
or
Photo 1
Photo 2
Photo 3

Should be fairly easy procedure.

Note: The photos are from a 1954 truck which came with vent panes. I assume your 48 does not have vent panes.
The doors that have a vent pane, the stainless steel trim is cut slightly different to accommodate the vent pane frame.
I believe both are attached to the door the same way.

1954 Chevrolet 3105 Deluxe Panel Truck
w/Factory HYDRA-MATIC