Turn off the power to the light circuit at the home's circuit breaker panel.
Loosen and remove the thumbscrews holding the fixture onto the mounting plate installed on the light fixture outlet box. Lower the fixture and loosen the wire nuts connecting the home wiring to the light fixture. Remove the light fixture.
Loosen the screws holding the light fixture box to the ceiling joist above the surface of the ceiling. Pull the light fixture box out of the ceiling.
Enlarge the hole in the ceiling to accommodate the recessed light fixture, using a keyhole saw.
Connect the bare ends of the recessed light fixture's wiring to the home wiring, matching the colors of the wires. Slide wire nuts over the ends of the paired wires and twist until the wires are tightly connected.
Place the recessed light fixture into the ceiling and attach it to the joist of the ceiling with screws placed through the holes in the light fixture's bracket.
Snap the trim ring onto the bottom of the light fixture. Place a bulb in the fixture and turn the power back on.