Disclaimer - if you aren't confident about doing this repair, call a repair specialist in. Also, take care not to damage the freezer coils as they have refrigerant in them that I'm told is harmful if it leaks. It may also be under pressure. The instructions that follow won't require you to touch the coils. There is no need to unplug the refrigerator or disconnect the water supply to do this repair. The screw driver you will need is a phillips (the one withe the + head).
1st, take out your frozen food (which, if you need to do this repair may not be frozen) and put it in a cooler or the refrigerator.
2nd, remove the ice cube bucket and dump the ice cubes in the cooler to help keep the frozen food cool. Turn the ice cube maker off.
3rd, remove the freezer shelves.
4th, locate the panel covering the freezer coils. It's a panel at the back of the freezer. It hides the freezer coils and protects them from damage. It's held in place by two screws located at the top left and right corners of the panel.
5th, unscrew the screws holding the panel to the refrigerator.
6th, detach the clip at the top of the panel.
7th, remove the panel. You should see the coils now. The coils will be covered in frost if the defroster heater has failed. If the coils are not covered in frost, there may be another reason for the refrigerator failing to cool.
8th, even if the coils are covered in frost, you should also inspect the defroster heater at the bottom, underneath the coils. The business end of it is a long tube running horizontally between two wires. If it's black like a badly burned out light bulb, then it's failed. If the bulb appears fine, you may have a different reason for your refrigerator and freezer frosting up. We'll assume that the defroster heater has blown out, and proceed.
9th, place a towel at the bottom of the panel space, to soak up any water that melts and runs down. There is a drain under the coils. It collects water that melts during the defrost cycle, but I'd rather mop up the water instead of relying on the drain.
10th, use hot air blowing from the hair dryer to defrost the coils. Just play the air from the hair dryer over the coils, starting at the top, and working down. The frost will quickly melt and the water will be soaked up by the towels. If you want things to go faster, you can throw hot water on the coils to melt them faster, but that produces more water to mop up, and makes a big mess. It's also dangerous to have water lying around if you then decide to use the hair dryer to continue defrosting the coils.
11th. MOST IMPORTANT. Don't use your screwdriver or any other sharp object to pick away at the frost on the coils. If you damage the coils the refrigerant could leak out. I'm told that it's dangerous stuff. Furthermore, I expect that you'd have to buy a new refrigerator if you damaged the coils.
12th, once you've defrosted the coils, locate the defroster heater and bracket assembly again. It's held in place by two screws, and has two insulated wires running into it on either side. Unscrew the screws, disconnect the wires and remove the assembly. Throw it away - there are no useful parts in it.
13th, install the new defroster heater and bracket assembly by connecting the wires to it, mounting it back into its place, and rescrewing the screws.
14th, put the panel back in place, replace the clip, and screw the panel back in place.
15th, turn the ice cube maker back on and replace the ice cube bucket and shelves. Put your food back into the freezer.
16th, use the freezer and refrigerator as normal. If the defroster heater fails again (I'm on my third one in less than six years), you'll notice frost building up on the panel at the back of the freezer before your freezer and refrigerator stops cooling. The frost build up is your signal to defrost the freezer and buy another defroster heater and bracket assembly. I don't know why this part is so weak, and don't know any fix except to keep replacing it.