How do you make the bad smell of the cage of a hamster go away?

You'll want to change the bedding at least once weekly. If there are places without bedding, you can wipe with a paper towel. I do this daily in a plastic "condo" that is so small, bedding just won't work in there. My hamsters nest in a small plastic dome (it was intended to be the "outhouse" and so I change the bedding there every other day. I have no smell. Good luck!