Remove the shower door from the track. Typically, the door will lift up into the top track enough to clear the bottom track.
Place the door flat on the work table. Mark the four corners of the glass door with a marker so that you can check your fit when you reassemble. Unscrew the screws holding the corners together.
Place a block of wood along the inside of the frame. Tap the wood firmly with a mallet. Move the block from frame to frame to loosen and knock the frame apart. You may need to use a lubricant on your corners and screws if they are rusted.
Slide the door away from the spline. Slide the spline away from the frame. Use your spline to order the correct type of glazing channel for your door.