yikes! BACKUP BACKUP BACKUP! The number one most important rule in updating.
Okay, so you do not have a file backup, but you do have your database intact at RC1, correct? PLEASE immediately backup your database now to avoid any further problems, we can then work from there.
The files themselves are not a big deal because you will basically be replacing all your files with phpBB 3.0.5 files. But for the sake of the update script, you will want to download the phpBB3 RC1
files and replace your current files with then. Ensure you don't overwrite your /files/ (attachments) or /images/ (avatars and others) or your config.php file. Be sure to backup your files and database first.
Next, download the RC1 to RC2 updater
Run the update.
Go to the phpBB3 Download files
, you'll want to download the 3.0.RC8 to 3.0.0 package, and run the update. It will update the database from RC2 to 3.0.0 stable. At this stage, download the 3.0.0 files and completely replace all of your files again.
You should then be able to download the next package, the 3.0.4 to 3.0.5 update package, again run the update and it will update your database from 3.0.0 to 3.0.5, and again replace your files with files from 3.0.5 and you're up to date!
Let us know how it goes.