"Off" button on Triton Opal has stopped working. How to repair?

Triton Opal Electric Shower has "On" buttons that operate the shower at varying temperatures, once the main switch has been put on by means of the cord. To stop the shower, a larger "Off" button, towards the top of the shower panel is usually depressed until the shower stops. This no longer works and the only way to stop and start the shower is by means of pulling the main cord / on / off switch. How can I repair the "Off" button?