Replacing it is pretty simple.
Before shutting down the PC, open Device Manager (Control Panel>System), click on display, then rt. click on the Nvidia 630, and then click remove. Answer yes if it asks if you are sure you want to do that.
Shut down the PC, (power it down all the way) , open the case, remove the 630: be careful, there probably is a small tab on the PCI-E slot that locks it in and that has to be disengaged before you can remove the card. Then insert the 970, secure it to the case just like the 630. IT will need to mount to two of the slots at the rear, unlike the 630 which is a 1 slot card.
Connect the two power leads and you are good to fire up, I usually cancel any installlations by Windows and manually install the Nvidia drivers using "Custom" and selecting "Clean Install".
HOWEVER, you have given us NO details about the system and that can be important.
Per Nvidia the minimum recommended power supply is 500 watts with a minimum of 28amps continuous power @ 50C; this is for the entire system. The power supply will also need to have, for one GTX970, two 6 pin or 6+2 pin PCI-E power connections. Some non-reference cards require a 6 pin plus an 8 pin.
Though the card will never draw that much power, the connectors for the video card, and sufficient amperage to run the card and the rest of the system is critical.
Does his system have enough power?