Option 1
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
Option 2
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