In September 2016 I bought a Pi-top with its Pi3B, which I assembled and it runs with the Pi-top software recommended at the time. I recently bought two Pi3B+. Expecting complete computability, I simply swapped the PI3B for a Pi3B+, using the same SD card and same everything else. However, it did not boot. I got the multi-coloured screen the normally briefly occurs, and the red led, but then nothing else. I tried the other Pi3B+ with exactly the same result. Putting the original Pi3B back made everything back to normal. Is there any reason my simple swap should not work? Thanks.
For the Pi3B+ you need to use the updated Raspbian image dated 2018-03-13.
You can try the following which may work for you. It didn't work for me as I don't use the default repositories.
sudo apt update
sudo apt upgrade
sudo apt dist-upgrade
As Joan and OneCookieLeft's earlier answers say, it is because you need a newer version of the image (https://www.raspberrypi.org/downloads/)
I didn't find the apt methods helped in my case and in the end opted for a fresh image.
However you may be want to consider this option: afew posts on the Raspberry Pi blog suggested that it might be possible to swap in the new Pi 3B+ if you use rpi-update on your existing card (after backing up your card first!) https://www.raspberrypi.org/blog/raspberry-pi-3-model-bplus-sale-now-35/#comment-1415071
There's a bit more detail on rpi-update here: https://raspberrypi.stackexchange.com/a/4363/82488