how can I fix caroma cisterm 413298 type 5?

Dear all,
my tolet is overflow the water . obviously it is leaking and the washer needed to replace. I open the cistern lid it was caroma 413298 type 5. I found it is very diffcult to disassemble the outer valet and inter valet in order to change the bottom washer. any body have any ideas how to do it? I'v struggled for few days for it! any infos is appreciated!