The Lens Distortion effect can be used to increase or reduce lens distortion. Add a lens distortion layer by clicking the Lens Distortion icon on the Effects tab of the toolbar.
Usage
Use the Pincushion/Fisheye slider to adjust the type and strength of the distortion.

Parts of the image can get clipped after distortion. Use the zoom slider to scale down the image and make the clipped areas visible again.