I have Intel Pentium D945 processor and a heat sink above that and a fan on top it!!..
The fan sucks the air from out of the cabinet..
Will making it blow air outside be beneficial for my system?
If yes, then what changes should be made to blow the air?
Do not change the CPU fan, it's doing exactly what it's meant to be doing. The best thing to do is make sure that the hot air generated inside the case is removed, by adding a fan to the inside of the rear of the case (depending on the case). You could also add a fan at the front of the case to draw cooler air from outside into the case. What case do you have? what model PC is it? It might give us a clearer picture of what modifications you might be able to make.
I believe you should keep the heatsink fan to be......blowing...air on the heatsink to draw the heat away. Now it would not be good to have all fans to be intakes because there would be too much hot air within the case. Instead balance it out...for a normal setup on low end cases it should be something like the front fan as intake (blowing air inside) and the rear fan (sucking air out of the case).