You need to make sure your power supply can handle it, and has the right connectors. Then make sure you ground yourself by touching the case or by having an anti-static strap to your wrist. When removing the old GPU, you should find a clip in the back of the motherboard slot that holds it in. Disable this and pull the GPU out.
Be sure to seat the new graphics card into the slot fully. You should hear a click from the hold down clip. Apply the proper power supply cords to the external power ports on the graphics card.