Edit one of Annual reminder impossible :-(

I have a reminder for a birthday. As I can only one "reminder" I set it for 3 weeks before to remind me to get present. I then wanted to set it for 2 weeks before but only this up and coming birthday. When I go to web site it says "do you really want to edit whole set" but there's no "No just this one occurence" option :-(

I realise i could have replied to Sandy's twitter with "Remind me in a week" but there's no way of seeing this is there? And there's no way of doing this if you inadvertantly deleted the twitter or email :-(