Step one, download the newest Nvidia drivers for your version of Windows. Uninstall the previous card's drivers. Goto to step 2
Step 2. You'll need to open the computer case, when it's off and unplugged of course. Remove the current videocard, usually there a screw holding the metal bit on the end to the case, and maybe a plastic latch on the other end of the card that holds it to the motherboard. Install the new card into that same slot, latching it in and screwing it down, just like the other card was.
Step 3. Boot up and install the new Nvidia drivers.