That's only if you installed one of the previous versions of Corona. Just go to Cydia > Search > Type in Corona > Tap Corona 5.9.1 Untether > Install > Reboot
Well he asked how do I update, so I assumed he had already had it installed... but nice instructions.
For what it maybe worth I just applied Corona 1.0-4 over my redsn0w 0.9b3 jailbroken iPad, and rebooted. Download iBooks from the iTunes store and open it without issue.