with Nock ring pliers
If you dont have any you can use a small screwdriver and place the head in the split of the nock ring and turn to open the nock ring wide enough to slip it off.

be very careful not to damage your string or serving.

the best thing to do would be to go to youre local archery shop and spend the 7 dollars to buy a good pair of nock ring pliers and while you are there get a spool of number 2 serving thread and tie youre nocking points on instead of using brass nocking rings.