Q.Indoor Plants Turning Yellow
What must be done to stop it?

Certified GKH Gardening Expert
Unfortunately, there are a lot of things that cause this, and many of them depend on the specific plant in question.
Generally, under/overwatering, underfeeding, too little or too much light (depending on the type of plant), too little or too much water, pests, and many other factors can cause this.
Unfortunately, without pictures or knowing the exact plants in question, I can not be of much assistance, here.
https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm