I have a fowlerware (rebaware) toilet. It is continuously leaking?

I have a fowlerware (rebaware) toilet. Water is continuously leaking from the back of the tolet bowl. Even after a flush water continuously keep running into the bowl. I believe it is the O ring not closing properly at the base of the pump.

How can I gain access to the O ring. Can you please tell me the process to follow to gain access to the O rubber ring to replace it.

Tks