How Do I Replace a microwave fuse?
The first question you really need to ask is: Why would I ever replace a microwave fuse?
From my experience, the most frequent reason that your microwave just suddenly stops working is because the fuse inside of it has blown. In most cases this happens after a long duration of high output cooking and then suddenly, the microwave just turns off as if someone has unplugged it from the wall. There are no lights on inside the oven or on the control panel and it is totally unresponsive.
People who have told me this story say that their microwave quit working and so they had to go buy a new one. They are shocked when I tell them that it could be repaired for under ten dollars!
Ceramic Fuse
The fuse that you will be replacing is a ceramic fuse. They are generally about 1-1/4" long by about 1/4" in diameter; similar to the one shown in the picture to the left. It is VERY important that you buy a replacement fuse with the same current ratings as shown on the old fuse or in the owners manual of the microwave. The fuse is rated by two values; the voltage and the current. It is possible to replace the fuse with a HIGHER voltage rating than the original because the fuse is designed to operate at a voltage in excess of what is available from the circuit of the microwave. You must NEVER replace the fuse with a HIGHER current rating (Amps) because the electrical circuit inside the microwave is not designed to operate at values greater than that.
How Do I find the fuse ratings?
You have to look closely at the metal ends of the fuse to see the printing that is there. If you hold the fuse vertically, the ratings are printed around the circumference of the metal cap. You should see text like "125V" and "15A" which would indicate that the fuse is rated for 125 Volts and 15 Amps. You might also see "125V" and "20A" which would indicate 125 Volts and 20 Amps. A tip to you before removing the fuse: don't scratch the metal ends of the fuse too much trying to remove it from its holder or you may have difficulty reading the rating when you finally get it out. Have a look at the close up picture to the 
Ceramic Fuse Close Upleft and notice the markings of "250V" on the fuse cap as an example of what I was trying to explain.
Most of these microwave fuses are readily available from your local hardware stores. I found good selections from Home Depot and from Canadian Tire.
The next Question is How Do I find the fuse in the microwave?
Every microwave oven is assembled a little bit differently so there isn't a simple straight forward answer to this question. The best advice is to first consult your owners manual to see if they have provided any direction in replacing the fuse. It's been my experience that generally the homeowner has lost the manual and so in that case you need to be a bit intuitive and look for some screws around the sides, back or bottom that look like they may be holding the outer cabinet to the inner frame. Now I have to stop you right there because there are a few safety items that you need to be aware of before you even take the microwave off the shelf. A microwave is an electrical appliance which has the ability to cause you harm if you aren't careful! They can be heavy and awkward and I've got a few tips for you to properly manage your beast so that nobody gets hurt (not even the microwave).
Please check out this helpful video on how to replace your microwave fuse: