From ModMyiFone - Wiki
Jailbreaking 1.1.3 is extremely easy now.
You must be on a jailbroken 1.1.2. Open up Installer.app, and under the System category, install Official 1.1.3 Upgrader. The app will download and appear on your homescreen. Press the Home button, allow the SpringBoard to restart, slide to unlock, and run the Upgrader app.
Let it run (it will take about half an hour or more), and then your iPhone will restart, and you're all set - that's all!