How do change from Mac OS X 10.4 to Mac OS X 10.5?

I have Mac OS X 10.4 (updated) but most of the applications need Mac OS X 10.5. How can I change? Where can I download the things needed? If I change will I retain my files and apps?How do change from Mac OS X 10.4 to Mac OS X 10.5?
You have to buy Leo. Don't do an upgrade. It will mess stuff up. Do a clean install and reinstall your apps.



You can also install Leo on a firewire hd and run it from there and keep your old system intact.



http://www.amazon.com/LaCie-FireWire-Ext you can share this drive with pc's, but you can only boot to OSX using the firewire port.



1-800-MY-APPLE they are selling Leopard again.How do change from Mac OS X 10.4 to Mac OS X 10.5?
You actually need to purchase the operating system install disc for Snow Leopard.