If you are installing a replacement trackpad that didn't come with the attached flex cable, you'll need to use the cable from your old trackpad.
To remove the flex cable from the old trackpad, first flip open the latch on the ZIF connector, opposite the cable. Then, apply a little heat from a hair dryer or iOpener to soften the adhesive securing the cable to the trackpad, and carefully peel off and remove the cable.
When installing your new trackpad, routing the cable through the upper case and past the logic board may be difficult. It is helpful to use the tip of a spudger to guide the connector past the logic board while pushing the cable through its slot in the upper case with your other hand.