Vitra 2 button toilet cystern?

I have a small leak running down the back of the toilet pan and I suspect a worn washer or gasket ? How to replace it ? I have removed the top and exposed the flush unit inside ,but cannot open the barrel? Can anyone comment or advise me how to check it out ? Thankyou. PS Thankyou to the person who explained how to remove the 2 flush button unit. Easy when you know how ! Tony