powermac g5 update 10.4 to 10.5

I have an old desktop mac that I have purchased Microsoft Office 2011 for; unfortunately, I have discovered that this requires OS X 10.5.8 or later to operate. This seems like it should be simpler, and perhaps I'm missing something somewhere, but I need to update my computer to 10.5.8 and I can't figure out how / where to go to do this. I believe that this computer is compatible with this version of OSX.

Here are the current specs for this computer. Any help is appreciated.

MAC OS X 10.4.11

Dual 2.3 GHz PowerPC G5

5GB DDR2 SDRAM

Mac HD