I'm trying to remove the CPU.
What is the black plastic round thing around CPU and how to remove it?
That ring is the mount for what is probably an aftermarket cooling fan.
If there are not screws visible at the top, try checking the other side of the mainboard (it will have to be removed from the case likely).
Looks like the cooler bracket - usually fastened underneath.
You'll probably have to take out the entire motherboard to get to it.