I have a Whirlpool over/microwave combo Model #GMC305PDQ07, how do you change the light bulb in the microwave?

yes. bulbs of microwave can be replaced.
Mention your brand/model/size and someone can help identify the right bulb and procedure to replace yourself.

See below an example Microwave Repair Replacing the Internal Light Bulb 40W Whirlpool Part...

I would start with the switch. Might be an adjustment. Microwaves usually have 2 or 3 door switches mounted to an assenbly. Sometimes these need adjusted. Still need to take control panel off to adjust.

You know how when you turn on a regular light bulb and it burns out with a bright flash? What happens is that when the filament burns out, it breaks and bounces back and hits the other electrical post and basically shorts out causing that bright flash. Sometimes, when 1 of the light bulbs in the microwave burned out, it shorted out, and caused the circuit-board trace to burn up. (This is not just conjecture; I've seen it.) If that's the case, then you would have to replace the main control circuit board. A rather expensive repair for no lights. BUT BEFORE YOU DO THAT, get an ohmmeter and check your new light light bulbs and make sure they are really good. Also, if you can, get a bright flashlight and check the light bulb sockets in the microwave (unplug it first). Check for possible corrosion or signs of arcing on the center electrode of the socket. If that's the problem, you might be able to use a small screwdriver to scrape off the corrosion of the socket (make absolutely sure you have your microwave oven unplugged before you do that!).

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please test this steps and fix it. God bless you

1. Light bulbIf your microwave light bulb is not working, perhaps it has blown. The first step is to replace the bulb and power on the microwave. If this does not correct the issue, the next step is to check the socket and wiring to verify if that is the problem.
   
2. Light bulb socket & main control boardIf your microwave light bulb is not working after the installation of a new bulb, check the light socket as well as the wiring to ensure they are properly operating. If these sources are functioning properly, the next step is to check for a faulty control board as a possible root cause to your microwave light bulb malfunction.
   

Either the wrong bulbs or just bad ones. The oven only gives it the 120 volts. It won't be the microwave oven. Just bad bulbs

My Whirlpool/Roper microwave/hood combo has the same problem. I replaced the bulb twice, checked the bulb and it works, but not in the microwave. I wrote to Whirlpool asking about the problem. Haven't heard from them yet.

There should be a panel covering the bulb iside the cookspace with a screw holding it on. Remove screw, cover then bulb.

If there are 2 bulbs try swapping them around. If the night light works then you just need a bulb. Take the bad one with you and match it up at a parts store. Some actually have a dimmer circuit programmed in the board and that will not be a easy fix. Do you hear a click when you try to activate the night mode? If you cannot find a bulb or do not want to replace or repair the unit just disclose the non working light. I was in real estate for a couple years and a burned out bulb will not be a deal breaker as long as they are told up front.
Eric

Most likely you're going to have to dismount the microwave and remove the outside cover to get to the bulb. This is a fair amount of work but is not rocket science. I'm not famliar with your particular microwave, but it's probably that way. If it's any comfort to you, light bulbs are usually not covered by warranties or service contracts.

It won't hurt to run the microwave without a bulb.