Generally when a leak appears at the top of a window, it is not a window leak. If you can actually see the water come between the glass and frame, that is a different matter.
I would venture a guess that it is either (1)poor or no flashing or (2) the water is coming from somewhere else and finding its way to the window because it is an opening in your wall. So long story short, don't replace the window until you know it's the windows fault or you may go to a lot of trouble and expense for naught.

thanks, but how would i go about determining the cause?

Open up the drywall above the windows where the leaks are happening.

I have vinyl windows that when it rain (mostly heavy rain), water stains inside at the lower right and left corners (some windows in the middle section as well as the right and left section). What can I use to stop the water from coming in (I know that the windows are not slant enough to remove all the water, I am seeing). I appreciate if you can help me out.

Many leaks around windows generally can be traced to a problem with your roof unless you can visibly see the siding or drip cap is not sealed correctly.

Drip cap or head flashing.