The previous poster was suggesting that you use your ftp client and copy the file you wish to change to your machine. Make changes to it and then ftp it back to your server.
Other option is to change the permissions of the files in your theme directory so that they are writable (chmod) by whatever process your web server runs as. Then you can use the built in Theme editor.
[Disclaimer — what follows is not gospel, just an attempt to simplify some security concerns. I’m speculating without any knowledge about how your server is setup…]
However, this tends to be a security issue as it may open things up so that any miscreant can edit them as well… It all depends on the permissions and ownership of the files though.
For instance, if the file owner of the theme files is you, and the web server doesn’t run as “you”, then you have to make the files writable to the world. Trouble looms… you’d want to change the permissions, do your changes and then change them back.
If the theme files are owned by the same process that your web server runs as, however, then you can just change ’em to 644 and be a bit more casual.
