if you have a LT model, There are two buttons to the right of the steering wheel, press both at the same time hold for a second, cycle through the option on your DIC until you get to oil life, press (i forget which button) to reset the cycle
My 08 LS XFE has 2 buttons on the wheel.^LS models are not equipped with those buttons. LT and up have them, the lowest of the low level cobalts (the LS) doesnt have it