Take lots of photos of each side of the original RCD/RCCB before you disconnect it so that you know and have a record of exactly where each wire needs to connect, and of course disconnect any mains power supply that you may have already plugged into the boat before you start.
When I fitted the new device on my boat, I worked mainly along the line of taking one wire off the original and connecting it into the identical location on the replacement as far as possible, but cable lengths or lack of it sometimes dictated that system would not work in all instances, and this is where those photos and notes that I kept, were very handy indeed.
Fortunately for me there was no question of whether the original device was malfunctioning or not as it had a large burn hole in one area.
The photo below was taken from my purchase history on eBay, and where you can see that much of the eBay search information I used was taken directly from the facing side of the device.