How can I remove the cable/cable head from the brake lever?

I'm trying to remove the heads of each brake cable from both levers. I can't push it out sideways without hitting the parking brake locking mechanism. The cables are removed from the body of the walker, as I'm replacing the drums and brake shoes on my Trionic. 

Do I need to remove anything else to have easy access to slide the heads out of their slots from the side?

In order to remove the cable/cable head from the brake lever, please do as follows:

1. Loosen the two screws that clamp the Veloped U-shaped grip bar (behind the rubber grip) and pull the grip bar out of the vertical 'grip post'. 
2. Loosen the small screw on the rubber grip 'clamp', and remove the grip. 
3. Loosen the Allen key screw on the side of the brake lever a little (max one turn), and you can slide off the brake lever from the grip tube. 
4. Detach the rear wheel. 
5. The main body of the brake lever, its cable adjuster screw and the nut on the adjuster screw all have a slot for the brake cable. By aligning these slots, you can unhook the old brake cable from the brake lever if you turn/pull the lever to the very end of its motion range (fully pulled). 
6. Attach the new brake cable to the brake lever as described above but in reverse.

Attach the brake cable end to the brake, and read the instructions here on how to centre the brake.