You cannot update from Mac OS X 10.4 to 10.5 on-line. You have to buy the DVD set to install 10.5. The Apple store (on-line) no longer has that version available. You can try 1-800-MY-APPLE and see if they have any, but most likely not. You may need to find a copy on places like eBay.
If you still have a PowerMac as indicated by your information bar, there is no where else to go with Mac OS X but to 10.5.
If you can find the disk and install, you can then do the on-line software update to get to 10.5.8, the end of the Leopard system.