Semi-related (I think), I want to change the settings (table_prefix and password, for example). Where can I change these?
In order to change the prefix you will have to rename all of the phpbb3 tables in the database (manually - although you may be able to write a script, or sql, to do it) and also change the prefix in the config file. To be honest it would be quicker to delete what you have and reinstall.
As for changing the password - if you have to ask that here then I would suggest that you leave it alone.