If your avocado tree is getting too much water, it could be causing the leaves to turn yellow! Be sure to let the soil dry 50% before you water each time! When you do water, water evenly around the pot until water drains into the saucer, then discard the extra water.
Avocado trees love bright light! If they are not getting enough light, their leaves can become discolored. Be sure that your plant is getting bright light with 6+ hours of direct light a day.
Yellowing leaves can also be a sign that pests are present! Inspect your plant closely for webbing, sticky leaves, or small pests and treat the plant with an insecticide if you see any issues.
Is your avocado plant showing signs of active new growth? This yellowing may be normal then. Plants use energy from older leaves toward new growth and this is a natural process. Feel free to prune the yellowing leaves once they are more than 75% discolored.