In some places, you will need to hang onto the edge of the cliff to slide down. 

In immersive mode

Approach the edge of the cliff and swipe down on the left side of the screen to hang on.

In standard mode

When you are at the edge of a cliff, swipe down with your joystick to cling to the edge.

Please note that if you have selected the ‘climb automatically’ option in the accessibility options, Lana will automatically pull herself up. Swipe down a second time quickly to let go of the edge and slide down.