Certified GKH Gardening Expert
Typically in houseplants, it is either underwatering or overwatering. Water when the top of the soil feels dry, but don't let the soil dry out all the way. This article will have some other causes for yellow leaves: https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm
As for when to change the soil, or repot, this article should help: https://www.gardeningknowhow.com/houseplants/hpgen/learn-more-about-repotting-houseplants.htm
Yello crunchy leaves? Typically, this is a sign of underwatering, and would occur on the older leaves. You describe "outside" leaves, however; could the leaves be getting burnt by the sun? Without knowing what kind of plants, what size, where located, the most general advice would be to always feel the soil, make sure the lower half of the pot is not too wet, and when you water, make sure water runs out the bottom of the pot. It can be very confusing for beginners because if plants are too wet for several months the roots die, thus the leaves get no water, and so "too wet" can look like "too dry". Re: ''changing soil," don't. The plants are given optimum soil and fertilizer by the growers, because they are interested in having healthy plants to sell. You dont have to repot until the plant is so big its toppling over, or you actually want it to grow bigger.