Click on the terrain.

Click on E for edit.

The terrain editor windows show up including one called terrain canvas. Along its top-right edge are buttons that look like a white square with a border and a black down arrow. Click on the white square and a page shows up behind the canvas.

On the right edge is a long bar fading from white at top to black at bottom. Along the right edge of this is a bracket.

Place the mouse cursor over the bottom part of the bracket. Left-click and hold then drag upward. The bottom portion of the long bar will turn red. The darker portions of the terrain canvas will also turn red. The lower portion of the terrain is clipped off. Slide up and down till you get something you like then release the mouse. Check the button on the far left of the screen to get out of the terrain editor and back to the bryce desktop.

Since the bottom of the mountain is now gone you will need to drop the terrain down to horizon level by clicking the down arrow below the E button in the main window.