It would depend on the modifications that you've made. My suggestions would be:
1. Make a complete backup of your existing files and folders as they are now
2. Download the latest version of the software
3. You did ask for the safest way yes? Then upload the new version (all except the uploads folder) otherwise you will end up overwriting any files and images that users may have already uploaded!!!!
THIS IS IMPORTANT
4. Run the upgrade (there should be no database changes) but run it as the web wiz software explains
5. The fun bit

now is checking the original files against the new ones and making any changes on the new files. If you were smart you made a note of what files you changed first time, if like me you have good intentions, but lose yourself in reams of notes, then it might take a few days or so to render the forum like you already have it. I use Ultra Edit and Ultra Compare, but generally the best way is to make detailed notes
Example:
default.asp - what did I change
You get my drift
That way, future upgrades may be quicker but you sound like you know what you may be doing anyway