It will be one of two ways.
1 The whole button housing will unscrew anti-clockwise and come out and then the lid will come off or
2 Press down one of the buttons and then get something under the other one and flick it out and then this will allow you to take out the other one and this will then reveal a centre screw which you will need to undo in order to remove cistern lid.
Either way don't go mad as you will end up breaking the flush button or it's housing, but one way or the other you will get a result.
bob
October 2009