Actually, it's a good question! The answer is that it all depends on which of the two available "one-click" install processes you used.
The "Easy one-click" is centrally managed with it's own server structure separate from your "normal" account. You do not have any ability to change the core files of the WordPress install in this instance, either via FTP or by editing the themes' code snippets in the WordPress administrative back-end. You are limited to the themes, and plugins, provided in this instance.
The "Advanced one-click" is a WordPress instance that is installed for you in your own directory space, and the restrictions indicated above do not apply. Wth this installation, you can customize to your heart's content, using SFTP, the shell, or the built-in WordPress tools. You completely control the code on the site as your regular account FTP user.
If you are not sure which method you used, return to the panel and carefully read the menu page regarding the one-click installs:
Install new website software - [color=#CC0000]Easy mode[/color]
(In easy mode, customizability is limited (you can only use the themes/plugins/languages we've included), but installation and maintenance is dead simple!)
Modify / remove previously installed software ([color=#CC0000]easy[/color])?
Install new website software - [color=#00CC00]Advanced mode[/color]
(In advanced mode, you have full control (the software is installed to your web space), but installation and maintenance is a tiny bit more involved.)
Upgrade or remove previously installed software - [color=#00CC00]Advanced mode[/color]
NOTE: You will not see the "Modify / Remove" (easy) or the "Upgrade or remove" (advanced) choices/options listed above if you have made no installations in that category.
More information is available if you click the "ajaxy" green arrows on the screen (where the asterisks are above) including:
By checking the respective "Modify / Remove" ( "EASY") and "Update or Remove" ("ADVANCED") sections for your WordPress installation, you can confirm which one you installed (your installation will show up in the section in which it was installed)
--DreamHost Tech Support