take out the power cables from the card and power supply, unclip the card from back of case, push down on clip at the end of the slot and it will slide out. After that, just put the new GPU in the slot (push but not with too much force) and connect the correct power cables.
That is if you don't already have a separate GPU
This is for just putting in a GPU;
just put the new GPU in the slot (push but not with too much force) and connect the correct power cables, make sure it is correctly connected to the back of case and is locked in the slot.