Einstein revealed that simultaneousness doesn't exists, it depends on observer...
This is even truer in computing, you have to update the cells one by one (unless you have 4 cells and a quadcore...) but computers are fast enough to give the feeling it is updated at once.
One of the secret is to do all the updates in the same draw() call: the screen will be updated only at the end of the function.
Funnily, a more often asked question is how to do such update in slow motion (eg. display a text letter by letter), which is harder...