You have done a backup right??
If you have, you have nothing to fear If you are truly paranoid then do a dump of the database as well. So you can go back if you have to.
I have upgraded loads of installations loads of times and never had a problem.
You don't say what sort of system you are using so its hard to give a blow-by-blow description. But roughly …
- Download the file and extract to a new directory alongside your existing "running" installation
- Copy the existing config.php into the new directory
- Rename the running version to "mymoodle-old" or suchlike
- Rename the new one to the name your running one used to have (if you see what I mean)
- Run http://myhost.mydomain/moodle/admin (modify for your site)
- Watch all the upgrade stuff go by on the screen
- Than't it!
If it does go horribly wrong, you can rename the old code back into play again and re-load the dumped database data - you won't need to though