If the Earth were to increase in diameter but have no change in its internal composition or mass, its average density would decrease.
The Earth's density is defined as its mass divided by its volume. If the diameter of the Earth were to increase, its volume would increase as well, but its mass would remain constant. Since density is a measure of mass per unit volume, the average density of the Earth would decrease as its volume increased.
To put it another way, if the Earth were to increase in diameter, its surface area would increase faster than its volume. This means that there would be a greater amount of empty space inside the Earth relative to its mass, leading to a decrease in its average density.