A quick way to tell where the power supply unit (PSU) is to take a look where the power cord is plugged in to the PC. The cord from the outlet will plug in to the PSU. Before you disconnect anything you need to make sure that all power to the PC is secured and the switch on the back of the PSU is turned off.
My recommendation would be to take the entire desktop to the technician so they can remove the PSU; they have the proper tools, grounding straps, and expertise to quickly, easily remove/replace the PSU. If they want you to do the removal, then let us know and we can walk you thru the process.