MacOS – How to update 10.4.11 OS X

I would go here 10.6.8 Combo updater from Apple and download it via wi-fi and then re-run it. This often will fix whatever issues occurred with a Software Update via the built-in app.

A word of caution is to have a backup before doing this.

Is there a way to revert in OSX?

If you have a Time Machine backup that would be the easiest way to revert.

Leopard, 10.5, had some interesting features but I wouldn't recommend upgrading if you're comfortable with your laptop as is. There's a good chance that your iBook won't run 10.5.8 as smoothly as it does run 10.4.11. As far as I can tell, this is your only choice to upgrade because your iBook won't run 10.6 (Snow Leopard).

The only reason I can personally imagine wanting to upgrade is to get any security updates that were issued for the last 10.5 release (10.5.8) but weren't for 10.4.11. There were roughly two years in between the last Tiger release and the last Leopard release, but I'm not sure whether or not any security updates were released in that time.