1. make a backup/zip of your local files (specifially everything in your phpbb2 directory.
2. Do a full database backup
3. Make a copy of config.php from your current forum and store it somewhere (you will have it in your backup in the worst case scenario but make another copy and keep it handy.
4. If you are overwriting all files (ie you currently have a standard board and you have downloaded the full package) then overwrite your old phpbb2 folder with the new one.
5. use your old config.php and overwrite the one in the new phpbb2 folder.
6. upload the entire folder
7. Once complete open your forum but point it to..
(edit as required)
8. Once complete delete the install and contrib folders locally and from your website.
9. Log in and check it's all ok
Patience of a Saint :: Memory of a Goldfish.