You'll need to do that before importing the images into Aperture. Aperture does not allow modification of the EXIF data.
EXIFTool is a command line EXIF data viewer and editor. It is very powerful and can be a bit intimidating at first but is worth the effort to learn it if you plan on doing a lot of work on EXIF data. I believe there are some GUI front ends, Google will let you know.
There are other programs out there again Google will do a more through job of listing them then I can.
Note it may be possible, if the originals (masters) are referenced to modify the EXIF data after an image is imported into Aperture. Never tried it so dont't know if the modified EXIF data will be read by Aperture. It would be an easy test to perform.
I think, what Frank meant, was to turn your managed original image file, that is inside your Aperture library into a referenced original image file. This is done by selecting the image and using the command "File > Relocate original/master" from the main menu bar. This command will move the original image file to a folder of your choosing outside the Aperture library, so you can access it from the Finder. After youapply your changes, you can move it back using the "File > Consolidate originals" command. Whether the command will refer to masters or originals will depend on your Aperture version. What is your Aperture version btw.?