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iMac G5 running OS X 10.3.9 and wanting to update to OS X 10.5.8, yet not working?

I've recently got a hold of an iMac G5 and it's currently running OS X 10.3.9, and I'm trying to update it to OS X 10.5.8, but it's not working. From this link, Mac OS X 10.5.8 Combo Update, I have downloaded an update but I'm pretty sure I'm not doing it right, as after downloading the file and trying to open it, I get a message saying there is "no mountable file systems".

I am new to the Mac interface, but I've had an iPhone for years. What am I doing wrong? I've got no software disks or anything to go with the computer, so is there any other way to update it?