To start, you will need to unscrew the graphic card from the back panel.  Once it is free, you can gently pull it out of the motherboard slot.  Be careful not to damage any of the connectors on the card or motherboard.  If you are replacing your old graphics card with a new one, you will now need to install the new card in reverse order.  Make sure all of the screws are tight so that there is no movement between the card and motherboard.  Finally, reconnect any cables that were disconnected and restart your computer.