You’ll find the hip zone adjustment underneath the bed.

You can fine‑tune the firmness both during assembly and at any time after the bed is set up.

Softer

Place the sliders towards the middle. The bed gets softer and your hip will sink further down.

Firmer

Place the sliders towards the edges. The bed gets firmer and your hip will be placed further up.

More information here:
Wonderland adjustable hip zone