Unfortunately, the picture is too small so see the problem in close up. Yellowing leaves can be caused for various reasons. Firstly, too much water. Secondly, a white palm scale that attacks the underside of the leaves leaving yellow blotches on the upper side of the leaves. You will see small brown scales on the underside. If there are only a few these can be wiped off, otherwise regular spraying of white oil or a systemic insecticed will help. The other issue could be rust or similar fungal disease, if the problem is fungal you can spray the leaves with a fungicide, potassium phosphite is a fungicide that will not harm environment.