You need to follow the cable and see if it goes to a driver box, or if it goes direct to the supply.
Your new light will replace the whole fitting that you already have. So you'll need one that has the same cut-out size and the same bezel size as the old one.
Also you really need to know the wattage of the existing ones, or your new one will be brighter, or dimmer than the others. And that will not be a good look.
You will need remove that can from the decking to fit the new one as TTC points out. That will only be suitable for the existing, and its a 99% chance that the one on TLC is not the same (despite looking visually similar).
Is there no crawl space under your decking? If you cannot get it out from above, you may need to lift a decking board (if no crawl space)