It can be confusing to know the difference between the two. When you say fiance, this is usually considered to be more masculine. This means that you will be using this spelling when you want to refer to “a man who is engaged.” When you say fiancee, this means that you are referring to “a woman who is engaged.”
That is the only time when you would need to change the spelling of the two. It is important that you will know the difference so that you will not make any mistake especially when you are writing it down. A lot of people may not know the difference until they are corrected by others who know.