you've got os6.0.0.600 installed and you want one of the earlier versions? if you specify which, it would be easier to direct you. you can find earlier official versions at BlackBerry - BlackBerry Desktop and Device Software Download Sites. as with any official version from a carrier other than your own, be sure to delete the vendor.xml file before uploading it to your device.
another resourse would be BlackBerryOS.com - BlackBerry News. the downloads tab takes you to both leaked and official versions of software. i think you have to be a member of that blog to download there, though.
I want to go to the previous o.s. I have the os6 now and I want the one before this but can't find it anywhere. Can anyone help? TIA
You can downgrade to a previous version of OS6. For example, you may have 6.0.0.600 loaded on your phone but prefer and earlier version. You can revert back to 6.0.0.448 or anything else.