populate a "multiple choice" dynamically...

Is there a way to populate a "multiple choice" element dynamicall? 

I dont want to reset it - I would like to change the entries....  and then if possible create new ones..

the Funtion "item" returns an element but I could not use it to "modify" an existing one ("write"). The notation mit [] doesnt seem to work (or I am doing it wrong!... which is very possible :-). )

Thanks