Upgrading from iPhoto '09 to iPhoto '11 is actually pretty easy. If you purchase iPhoto as part of iLife '11, just run the iLife '11 installer. If you purchase iPhoto '11 from Apple's Mac Store, the software will be automatically installed for you.
But there are two things you should be sure to do; one before you install iPhoto '11, and one after you install it, but before you launch it for the first time.
Back Up iPhoto
Before you install any iPhoto upgrade or update, you should back up your iPhoto Library. This is especially important with iPhoto '11. There was a problem with the initial version of iPhoto '11 that caused some individuals to lose the contents of their iPhoto Library during the upgrade process. By backing up your iPhoto Library before you upgrade iPhoto, you can copy the iPhoto Library backup file to your hard drive if something goes wrong during the upgrade process. When you relaunch iPhoto '09, it will update the library, and you can try the upgrade again.
If you're not sure how to back up your iPhoto Library, our Back Up iPhoto '11 - How to Back Up Your iPhoto Library guide will walk you through the process. (The instructions are the same for iPhoto '09.)
After you upgrade iPhoto but before you launch it for the first time, use Software Update (Apple Menu, Software Update) to check for updates to iPhoto, which is currently at version 9.1.1. (Although iPhoto is part of the iLife '11 Suite, it's actually iPhoto v. 9.)
If you prefer to perform a manual update, you can download the latest version of iPhoto at Apple's iPhoto Support site. Just click the Downloads link.
Be sure to update to the latest version of iPhoto '11 before you launch iPhoto for the first time.