The two above just explained it.
Like we said, just pull out the 7770 (make sure to undo the pci clip first on the right side of the slot, don't snap it like my friend did)
push in firmly the 970 and push the clip back, and plug the 2 power connectors from your psu to the card.
From there your card is now almost fully operational, now you'll have to go the Nvidia website and download the gtx 970 drivers so that your gpu can be utilised by programs such as games, also you may want to also remove the AMD driver (although not necessary).
if you need more help on psu power connectors look here:
http://www.playtool.com/pages/psuconnectors/connectors.html
At the top of the page it shows images, that may help you more.
Anything else you're unsure of?
-good luck