Timer is located inside the lower right side, near bottom, of the back of the machine.....you will see a couple of 1/4 inch screws that secure it. You should see the hole where you can insert a small regular screwdriver to advance the timer. Before taking freezer apart set your amp meter to six amps and place it around the yellow wire coming off the timer and advance the timer till the you feel a click and the fridge stops running.....now the machine is in defrost. If you get a reading on the amp meter that tells you everything in the freezer is ok and just change out the defrost timer....if you don't get a reading then the heater or bi-metal in freezer is bad and you have to take freezer apart to check those components. Always change defrost timer when replacing other components.
It's hard to describe how to take the freezer apart on a message board, but the freezer floor pulls out after taking the divider (metal strip between freezer and fridge section) is removed.