Unless you live under a rock, you'll already have heard about the release of Joomla 1.6. You can read the full Joomla 1.6 Release Announcement here. It seems the hottest topic as a result of this is: "How do I upgrade to Joomla 1.6?" Let's just provide a bit of background information to you before we get you started on the answer to that question and provide you with some more Joomla Tutorials.
First of all:
Why do you want to upgrade to Joomla 1.6?
You should consider using Joomla 1.6 for your new Joomla sites in the near future, however, with regard to your existing Joomla 1.5 sites, why do you need to upgrade? Joomla 1.5 is a long term support release. This means it will be supported with security and bug fixes for far longer than Joomla 1.6 may be. Note too, Joomla 1.6 is NOT an upgrade from Joomla 1.5, it's a MIGRATION.
So, how do you migrate to Joomla 1.6 from Joomla 1.5?
There is work being done on a migrator component that will assist you to do this. Probably the easiest way to show you the steps involved is to point you to the following short video:
You can read more about this process at the following site: http://docs.joomla.org/Migration
We hope the following information helps you out for the time being, and remember.. Joomla 1.5 to 1.6 is not an upgrade, it's a migration. That being said, it's not something all users will need or even be interested in. The future is Joomla 1.6 and beyond, but for existing sites that already work well, stick to Joomla 1.5.