There are several things that cause yellowing leaves. Can we get more info? This info will help diagnose:
What strain is it?
Is it in Veg or Flower stage?
If in Veg, for how long?
If in Flower stage, For how long?
Indoor or outdoor?
Soil, Soilless or Hydro?
If soil/soilless, what is in your mix?
If soil/soilless, what size pot?
Size of grow space (LxWxH)?
Make and model of grow light?
Light distance from the top of the plants?
Temperature in the environment?
Humidity in the environment?
PH of medium or reservoir?
EC/PPM of medium or reservoir?
Any Pests?
How often are you watering?
Type and strength of nutrients used?