LED resistor
The resistor takes the voltage left over after the LED. More resistance means less current, not a change to the supply voltage.
R = (Vs − Vf) / I Pᴿ = (Vs − Vf) × I
One LED, one series resistor, a DC supply and a constant Vf model. Verify worst-case supply, minimum Vf and resistor tolerance. Choose a suitably derated power rating above the dissipation. High-power LEDs usually need a proper driver.