Why do you even want to?? If you mess with it, the file will loose it's identity to the Mac OS (as in the OS won't know what application to use to open it with). Chances are, the name will also become truncated to an 8.3 convention and show as a pc file. We had a server that was stripping resource forks from files, which made them react just like that.
Mess with it at your own risk. IF the file is not showing up properly, such as with a generic icon, then boot into OS 9.x and hold down the option+apple keys before the desktop starts loading (after the file extension/startup item appears on the booting screen). That will rebuild the desktop, and will (most of the time) restore the file icon/association. Provided you haven't messed up the resource fork.
If you don't know EXACTLY what you are doing, and do go and try and alter the file/forks, I hope you do it to a copy. The chances are high that you will completely mess up the file (you could get lucky, but don't bet on it).
there are a number of applications that let you mess with this kind of stuff. One that has been popularized on resex 'explodes' the forks into a folder so you can edit the resources.
another app switches the data and resource forks. in osx apple has been phasing out the resource fork. so a lot of info that used to be in the resource fork is in the data fork. so you switch the forks, edit what you need, and then switch them back.
all of this kind of stuff can be found at resexcellence. (id give you some specific links but im kind of busy at the moment, i though you should be able to find them though).