how to reduce current to power LEDs
I'd recommend a low-side N-channel MOSFET with a PWM control line to control brightness. Basic schematic concept:







simulate this circuit &ndash; Schematic created using CircuitLab

When the PWM signal is high, the NFET conducts, current flows, and the light turns on. 
When the PWM signal is low, the NFET stops conducting, current stops flowing, the the light turns off. 

By varying the duty cycle (percentage of time the PWM signal is high), you can control the brightness. A simple microcontroller like an Arduino can provide the PWM signal. You'll want to look for an NFET that uses logic level signaling so the Arduino's GPIO pins can fully turn the NFET on and off.