usually you uninstall the old card's driver first, then shut down your pc, disconnect the power cables, ground yourself, open the case, carefully take out the discreet gfx card out of its pcie 2 slot, put the new card back in, close the case, reconnect the power cables, turn on the pc, install the drivers for the new card.
if you have integrated graphics, you can add a discreet gfx card. discreet gfx cards are usually more powerful.
what are your pc specs?
what is you budget?