How to replace the fan?

Delta Mike, download the service manual from here. you did not tell us if you are referring to the case fan, the PSU fan or the fan in the heatshroud for your CPU. If it is the CPU fan then all you have to do is to open the case, lift the shroud that covers the processor and remove the fan. Follow the power lead to the motherboard and remove the connector. If you look at the fan you should find some release tabs that lock the fan in place. Press the tab and it should then easily pull away. as to where to purchase it, that will depend on your geographical location. Hope this helps, good luck.