Square yards and cubic yards are units for quite different things, and you can't convert from one to the other. A square yard is a a unit of area and a cubic yard is a unit of volume.
Suppose you have an piece of your lot that is 4 yards by 3 yards then this is an area of
4 3 = 12 square yards.
If you need to cover this piece top soil and you need to know how many cubic yards to order, you need to decide how deep you want the top soil. Suppose you want 6 inches of top soil then since there are 36 inches in a yard, 6 inches is
6/36 = 1/6 yards.
Thus you need
4 3 1/6 = 2 cubic yards of top soil.