Easy: just unscrew the incandescent or CFL bulb and screw in an LED bulb.
Replaced 120 W incandescent bulbs in my partner’s office ceiling fixture with 28 W of LED lights
Replaced 84 W of CFL lamps over bedroom lav with 10 W of LEDs
Replaced 60 W of incandescent lamps in entry hall with 28 W LEDs
Replaced 32 W of CFL lamps on both patios with 30W LEDs
Somewhat Less Easy; first unplug the fridge and remove the plastic covers over the old lamps in both the ‘refrigerator’ and ‘freezer’ compartments. Then replace 120 W of incandescent refrigerator bulbs with (much brighter) 18 W of LED bulbs. Covers back on, fridge plugged back in and you are set!
Even Less Easy; After turning off the associated breaker, rebuild each fluorescent fixture as an LED fixture by removing the ballast and replacing the tombstone connectors and wiring. Breaker back on and hey presto! This is a “Really Avid DIY’er” task, not something the average person could do easily:
Removed 148W of garage fluorescents. Replaced them with 56W of T-8 LED tubes.
Replaced 80 W of fluorescent lamps in my office with 36 W LEDs
Replaced 136 W of fluorescent lamps in kitchen with 32 W of LEDs