Installer

Open iTunes
Click your iPhone in the navigation tree on the left
Click Upgrade, or Restore in the pane on the right

If you can't download via iTunes for some reason, you can download 1.1.4 here:
http://www.versiontracker.com/dyn/moreinfo/macosx/32939

You will then need to press and hold the "SHIFT" key while clicking the update button - an open file dialog will pop up. Now you can select the firmware you just downloaded.