changing pll settings

Try upgrading to a more recent version and you should have a much easier time. Is what users used to do was create a small project with the same PLL instantiated in it and a few registers for it to drive. Make the Megawizard settings that you want, compile, open the chip-editor and see what the low-level parameters are set to. If you want something else and can't quickly do it with the low-level parameters, repeat the process. Just a hint if for some reason you can't upgrade. (Although naturally, it is always recommended...)