An unfinished cup of coffee will never warm up on its own.
Maybe you chug it before it crosses the line from lukewarm to cold.
Or you pop it in the microwave, trying to bring some heat back at the expense of flavor.
You could always just brew another pot, and either add in a warm up or start with a fresh cup.
Whatever it is, you need to do something, or else hours later you’ll wind up finding a forgotten half a cup to pour down the drain.