In general, it's easier to remove food from silicone trays. Any shape works for JELL-O, but when it sticks to the trays or molds containing them, it can rip. After JELL-O rips, you can't repair it—it will only lose its shape. The best time to remove it is when it's completely cooled and formed, otherwise you risk causing a liquidly mess.
Step 1:
Put a sink topper in a sink to close the drain.
Step 2:
Fill the sink with two inches of hot water.
Step 3:
Hold the bottom of the silicone tray containing the Jell-O in the water for 20 seconds. Do not completely submerge the tray.
Step 4:
Turn the silicone tray upside down over a plate. The JELL-O will slide out onto the plate.