There are a few ways, with these solutions also depending on your computer.
If you have a Mac, you can use the Terminal program and dial in to your Dreamhost account, using your username and password.
If you have a Windows machine, you can use the Command Line tool ("cmd") and dial in the same way.
Also, you can log into your website using an FTP program. Depending on your computer (Mac or Windows), you can download a program that will help you do this.
The fastest way however, is to just log into your account, navigate to "Domains > Manage Domains" and you should see a listing for your site. Click on the "WebFTP" link and that will open a window where you can access all files in your site as you please.