You might find some help here using the Search box of the Forum. Unfortunately, the c100 series player is so old, it’s not even listed in SanDisk’s KnowledgeBase as a “Legacy” player. That pretty much means it’s prehistoric, so you’re going to have to dig to find anything on it.
Note, there is also Google which is a great resource for gleaning info from everywhere. Typing in “formatting a Sansa c140” reveals quite a lot. In fact, the 1st (top) result indicates you format it as you can with any other model; by connecting to the computer, right-clicking on it in Windows Explorer and selecting Format. 
