It is located below the images panel and lets you change the shapes of most models. You can scroll to it or open it by hitting the S key
Most models have shape adjustments. If they don't, they won't have a shape panel. Hit the reset button to return to the original shape.
Some shapes changes will make the art distort. You can compensate for this by stretching the layer in the opposite direction.
The stretched layer can be duplicated and replaced. The new layers will keep the same amount of stretching.