If you use an FTP client you should see an option to change the permissions of a file/folder. Usually you select the file/folder then right click to bring up the context menu.
If you use SSH to connect to your shell then use chmod 644 filename/foldername (more than one separated by a space).
Are you sure you know why you want to do this?
See chmod in the DreamHost Wiki here
Opinions are my own views, not DreamHosts'.
I am NOT a DreamHost employee OK!! :@
You act on my advice at your own risk!