Are the words won't and wouldn't interchangeable in sentences ? How are they properly used?

Won't is "will not."  Wouldn't is "would not." 

The difference is subtle.

Will is more concerned with an inevitable event.  Will not/won't has more of a "no way," connotation. 

Would is more of a condition or mood. 

However, there are occasions were "would" is the past tense of "will."  "He said he would do the dishes."