Unplug the PSU and remove the cover, and be careful not to touch anything more than what you have to. The PSU can still hold power inside once turned off. Unplug the stock fan and remove it. You should be able to swap fan connectors using a small flat screwdriver to get the pins out. Make sure you don't mix up the wires. Then put the new fan in.
Make sure your PSU doesn't get too hot though, as the NMB case fans have pretty low airflow, and you don't want your PSU getting excessively hot.