NOTE: this upgrade process works the same way for Magento version 1.4, 1.5, 1.6, 1.7, 1.8, and 1.9
As many magento users will know, Magento is no fun to work with-- mainly due to poor community support (the developers help paying customers; i.e. Magento Enterprise ... Only payed support there... Great for business but bad for the average or beginning user...)
So here's how we do it.
MAKE A BACKUP!
You can skip this step all together, however, please make a backup of everything prior to beginning the upgrade.
Create a test environment somewhere on your server (preferably away from your production installation)