First, your Power Supply should be about 500W or greater and have the proper 6/8-pin PCIe power connectors.
Assuming this, here's the instructions:
1. Remove the current video drivers from add/remove (uninstall)
2. Shut down PC, swap cards, and attach monitor to DVI (VGA if older monitor)
3. Start PC, then go to the main NVIDIA site for drivers.
4. Follow the instructions to auto-check your PC so it gives the correct version of drivers, or do this manually.
5. Install the downloaded drivers.
6. Adjust the resolution if needed (usually the monitor's max resolution such as 1920x1080).