it will depend on how your case works, first you have to unscrew it from the case (or if it is pressure fixed remove the pressing plate).
later depending on the motherboard... you have normally a lock on the PCI slot, some unlock by pushing a lever or pulling a plasting, it will depend on your motherboard version.
last but not least if you have any power connectors just remove them...
It is something intuitive, don't force anything, if you are forcing something is wrong...