https://www.rollitup.org/marijuana-plant-problems/157345-have-plant-problem-check-here.html
look thru this specifically at nitrogen, potassium, zinc they have things to do with yellowing leaves
next any info on the plant is always helpful were not in your grow room so fill us in
ie ,indoor, outdoor, hydro soil or other, soil, temp, nutes, feeding schedule, watering schedule, photoperiod, light type, strain or bagseed, anything that you think we need to kno tell us and someone on here can prolly pinpoint the problem and solution