How to remove/replace a Roca single pushbutton for the toilet cistern? ?

Does anyone know how to remove/replace a Roca single pushbutton for the toilet cistern?
I've just receive the replacement button as the old one broke and don't want to break anything by doing it wrong. How easy is it to replace it on my own or do I need to call a plumber? The kit contains a rather long plastic rod that I assume needs to be shorten or not???
Any advise is appreciated!