You mean removing the video card?
Just power off the machine, unplug the monitor cabling from the back of the card, then unscrew the end bracket that holds the card to the rear of the case. Then unplug it's power plug(s), then check to see if the PCI slot it's in has a "lock" tab on the end of the slot on the end away from the back of the case, if so, slide/pop it to unlocked position. Then gently start pulling the card out while rocking it a small amount, if needed, until it comes free of the slot. Install the back plate and reverse the steps you followed to remove it, and power the machine back on. There are no ramifications as long as you only add and remove the card with the power off.