Cleaning a Keurig machine isn't really a matter of washing it with soap and water like your other dishes. Over time, coffee oils start to build up and leave permanent smells in your machine. Have you ever noticed how your coffee mug eventually begins to smell like coffee event when there isn't coffee in it? The same happens to your Keurig, and other coffee makers for that matter.
There are some ways to clean it once it starts to smell, but the best way to avoid this is by doing some preventative maintenance. There are several ways to make sure your Keurig machine avoids smelling like old coffee in the first place.
How to prevent old coffee smell in your Keurig machine
Don't leave pods sitting in the holder. It's the same case with drip coffee makers. You shouldn't leave the filters with coffee sitting in the brewer after you've used them. When these pods sit in the Keurig for hours, they release oils that stick to the machine. Over time, as you do this, those oils are harder to wipe out, and will leave the Keurig smelling like those oils. Then, your machine starts to smell like all the old coffee pods that you have left sitting inside. Try to take them out as soon as you're done brewing coffee, or as soon as they cool off.
Run a cycle of just water afterwards. Baristas perform a similar cleaning process with their espresso machines. They run a cycle of just water through the portafilters (the handles that hold the coffee) after every shot. This will help get rid of some of the oils before they really stick to the Keurig machine.
Keep the drip tray coffee-free. Sometimes, the pod holder will drip some excess coffee into the tray. This will eventually build up and make the machine smell like coffee too. Be sure to check what is in there every now and then to keep it clear of leftovers, which can start to mold too!
How to clean your machine
Wash by hand. The Keurig machine is not dishwasher safe because the plastic is too delicate. Take apart the pieces and hand wash them with warm soapy water. Lay out to dry completely before re-assembling.
Descale your machine. Often time, we're not brewing coffee with completely clean or mineral-free water. This causes "hard water" to build up in the machine. Then, it can get so backed up that it stops working. Try to de-scale your machine every few weeks or so (depending how often you use it) by flushing your Keurig machine with Keurig's descaling solution or a mixture of hot water with distilled white vinegar.
Overall, it's really not that hard to keep your Keurig machine clean. You just have to make sure there's not coffee in it whenever you're not brewing coffee!