Shabdez: Aloy should grab those handles or ledges automatically once she (=her hands) get close to it. In most instances this would be by jumping towards the object, but every movement into the correct direction should work. If you are already standing on a ledge and want to go down by using the objects, slowly approch the ledge until a prompt pops up telling you to use the key for crouching.
To navigate up, down, left and right while you are in a climbing parcour, use the regular movement keys (forward = up, backward = down).
To release your grip, press crouch key. You will then simply drop down - only works like that if you have a free path to the ground level. If there is an other object right under your feet, you will grab that.

EDIT: i am not 100% sure, but child Aloy can not yet climb...

Keith_I2: I realize I must grab onto objects and ledges to climb, but what pc keys are used to grab objects and ledges, and then pull up?