What platform are you on (PC/PS/XB)? The key/button you need should be displayed along the bottom of the challenges window. Note that on console, some buttons do double-duty: if you see an additional circle around the button icon, it means you have to press and hold to get that specific action.
This one is stupid, Khajiit assumed you re-rolled all the options at once, like getting an new hand of cards.
As it is its much better and you can re-roll after doing some challenges