It is extremely easy to replace. So when the graphics card is in the slot, it is secured by the slot itself, you can see a little lever at the end of it. There are also screws that hold it in place at the side of the case on the inside, that is holding the bracket where the video connectors are. All you need to do is unscrew the screws, push the lever to open it up an take the GPU out.
Then put the new GPU in, it may be a snug fit but do not be afraid to put a little pressure to push it all the way into the pci-e slot. If the pci-e lock does not lock after the card is inserted make sure to lock it, then screw in the metal bracket to the case. That is all you need to do.
Like I said before as well, you will need a 6-pin power...