Been awhile since I use the 1-click install. If I remember right it checks if this is an upgrade or fresh install. Before you do anything, back up your database. You can also go to phpbb's website and read about upgrading there. The process is very painless especially if you did not add any mods. If you did add a mod, check to make sure you have the lastest update for that as well. Then you will need to check and make sure it is still installed correctly, odds are some of the files that the mod modified (pun not intended) have been replaced by the update, these files will need to either be replaced again by the mod version or manually modified with a text editor.
This is not really difficult, you just have to make sure that the line in the documentation that comes with the mod matches what you type and is in the right location in the file.
Hope this helps.