ive detailed the process here
but for most it will be easier to wait for a new 8gb image.
no you don’t need to unscrew the case, actually i tend to use another sdcard to pop the existing one out, its spring loaded, so it just needs a little push., then you can grab with your fingers.
replacing is easy, you can locate it in slot, then again use something to let it spring back in.
(makes more sense doing it, than describing)
… just be a bit careful when you insert, that you can feel its in the slot, its possible (i.e. i have done it 
) for the card to fit above the slot, and just fall into the case… then you’ll have to open the case to fish it out 
(it if does happen , don’t worry, opening up the case is no issue, just undo the screws, you wont need to take the board out to get the sd card rattling around inside ;))