A light-emitting diode (LED) system to retrofit a light fixture has a frame configured to be installed under an existing light fixture and a LED lighting assembly couplable to the frame. In one example, the frame is formed as a plurality of rails having a flange to be installed between the light fixture and a grid member of a ceiling suspension system. The frame may be installed without removal of the existing light fixture. The LED lighting assembly includes an LED array and an optical waveguide for dispersing light from the LED array. The LED system maintains a thin profile, enabling it to retrofit existing light fixtures. Installation time is reduced, thereby reducing labor costs, while the LED lighting provides more light with less energy.