Fan is making strange noises. How do I replace it.

My Gateway NV57H (which is just a rebranded Acer 5750) has been making strange whirring noises every time I have it open for ten to thirty minutes. It seems to be emanating from the fan. I just purchased a new fan for the laptop, but I have no idea how to remove the old one and install the new one. Does anyone know how to do this?