If at any point the case feels stuck, stop and look for any clips that may have re-engaged and release them with the opening pick to avoid damaging them.

Hold the main body of the controller with one hand.

Lift the rear case with your other hand. It should lift at an angle.

Pushing slightly towards the L and R buttons, lift the rear case off of the controller and remove it.

During reassembly, make sure the plastic covers for the adaptive trigger assembly adjustment sliders are correctly installed over the sliders.