A good safe way of removing thermal paste from a CPU is to use a paper towel and first wipe as much of the thermal paste off the CPU.
Then take some rubbing alcohol and pour a bit of it on the paper towel and then lightly scrub the thermal paste off the CPU.
Then wipe the thermal paste off again with the paper towel and it should clean the CPU off really good which is the way I do it myself.
You can also just use some baby wipes and clean the thermal paste off the CPU as well which is safe.