The simplest method would be limited LED output currents. There are 6 jumpers on MS610UB-V3.
From left to right J1P ~ JP6. Increasing the value of the resistor will reduce brightness level. For an Example: If you purchased LED kit Part number: UB54220LED5620X1+MS610UB
By default, JP2 shorted (1R5) on MS610UB-V3. If it is too bright. then removed solder on JP2, Shorted JP3(2R). It will be dimmer, or if it is still too bright, remove JP3 shorted JP4
If your application needs to adjust brightness level during day and night time. You need to add a 5~10K Potentiometer as Shown photo below