ok lets see if I can help
does the fan have four screws on it if so just unscrew tham and take it off and replace it with another fan. if not look below
when you open your case you are looking at the fan on top of the heatshink, does it have two metal leavers one on each side of the fan?
if so lift them up you will notice that they should lift up in different directions.
once you have done that you will have noticed that the top has come loose
look at the front and back of the heatshink and fan you should see where it clips into, each other
you will have to unclip the plastic brackets that hold it on, its best to do one side first then the other,
once you have unclipped them you will be able to remove the fan and the heatshink if you wish.