Look underneath your deck to determine where the floor joists are located. Generally, the joists will run perpendicular to the deck planks and be spaced between 1 and 2 feet apart.
Put on goggles and set a circular saw cut-depth to the same thickness as the Trex deck planks. For instance, if your deck planks are 1 inch deep by 4 inches wide, you would set the blade to cut 1 inch deep.
Turn on the circular saw and cut into the damaged deck plank lengthwise, just a few inches inside the deck joist. Cut through the Trex deck plank lengthwise, stopping a few inches before reaching the next joist.
Insert a claw bar into the saw line, close to one of the joists. Pry up and begin to break the plank into two -- the composite Trex deck plank should split evenly lengthwise. Continue along the cut line, prying up until one side breaks away length-ways.
Pull up the other side, unfastening the screws with a screw gun. Unfasten out any other screws still in the joist with the screw gun as well.
Measure the length of the plank you removed with a tape measure, then measure and cut a piece of new Trex decking to the same length with the circular saw.
Fit the replacement plank into the empty slot and fasten to the joists with a screw gun and stainless steel screws to finish.