In your main Civ2 folder, create a new folder called something like 'Backup', and copy the following files into it:
To uninstall Brave New World, simply copy and paste the contents of the backup folder into your main Civ2 folder, selecting 'Yes to All' when asked if you wish to overwrite your existing files.
These instructions are repeated (hopefully rather more clearly) in the ReadMe.txt included in bnw.zip