how easy or difficult is a fuel injecter to replace. on 2000 Ford F-350 Super Duty

I have been told by a mechanic that hooked up his tester under the dashboard that 2 injecters are not working. How can I determine which two are the bad ones?

Asked by for the 2000 Ford F-350 Super Duty
if you don't have a lot of experience, don't attempt. injectors are under valve cover. after replacement, glow plug is pulled and cylinder is cleared. if not, engine will hydrolock as diesel and coolant will drain into cylinder as soon as injector is pulled out.