I have already purchased airline tickets and paid for them with my credit card. After this i relized I had enough points to pay for my tickets. Can i now change this and pay for my ticket with my points and get a credit back on my credit card?
Assuming you've purchased a Wanna Get Away (non-refundable) fare, more than 24 hours ago, no.
If you cancel now and rebook using your points, you'll receive Travel Funds for the cancelled flight, good towards future travel for the original passenger only, valid for one year from the date you made the original purchase.